I’m very new to gaming. Have searched the web and can’t find an answer. There is a gym right down the street from me. Have never done battle or participated in a raid battle. Sometimes there are Pokémons there, sometimes not. But when a raid battle is scheduled there is always an icon of an egg in the lower left corner. Tap on it and I’m taken to what looks like a stadium with the egg in it. Can someone please explain? The best I can figure is the Pokémon in the gym are guarding the egg??? Thanks in advance.
Once there is previsted a raid, appears this egg in the left corner. It gives for the time until the Raid starts something like protection to the gym. So during this time you cannot attack the Mons inside the Gym and change this way the color of the Gym.
The Eggs appear at Random. Once they appear a timer starts counting backwards to 0 from 60mins.
Once that timer reaches 0 the Egg will Hatch for a chance to battle a Raid Boss for 45mins (another timer starts counting down).
The colour of the Egg dictates the Level of difficulty the Raid Boss will be.
In your example the Dark Blue/Purple colour is for a T5 legendary. You will need to have a group of other players to help you battle this Raid Boss. High Level Players can battle and win with only 4 players. Much lower levels may need up to 12 fighting it together to win in the time your given. You can have as many as 20 battling it together.
If you win the Raid you get a chance to catch it and keep it for your collection and/or use it to do further battle with. The contribution you make to the Raid will determine how many Premium (white) Balls you get to try and catch it with.
Any Pokemon that were in the Gym have nothing to do with that Raid Boss Battle that is going to take place.
The only relevance is the colour of the Gym. If its the same as yours you can get an extra ball for your tally to help catch the Raid Boss.
The Gym can battled and colour changed while it has and Egg on it but once that Egg has Hatched the Gym is locked from normal Gym interaction apart from spinning the Disc.
Once th Raid Boss timer is finished the Gym returns to normal Game play.
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Better answered than I…
Thank you so much! So the icon is just an “egg timer”. Sorry I had to. Now let me make sure I understand. There are 2 ways to battle at a gym? One with the Pokémon who appear in the gym, and one with the raid boss that hatches from the egg?
Thank you. And yes, your response did help. So, if I battle the Pokémon in the gym, and I win, I can change the color of the gym to the color of my team?
Exactly, in the Raid with the egg you have ONE enemy you can catch. In the other case you can have between 1 and 6 enemies, but you can win only XP and after you can put a mon of yourself in the gym.
Now there are no mons and no egg. Do I just stroll on up to the gym and somehow put one of my mons in there?
Depends on which color hase the Gym. If it has the same than you, and there are less than 6 mons, than you can add one of yours.
If the color is different to yours, than you can fight the Mons in the Gym, and if you win them all, then you get a price and you can put one of your mons. So the Gym changes the color to yours.
IF it is white no team owns it an anyone from any team can put there pokemon in it
A good detail to add… 
Once the egg haches, a Pokemon will occupy the gym for 45 minutes, and you cant challenge the defending Pokemon, feed them, or add yours
All you can do is spin the stop at the top and try to battle the boss using raid pass
The raid boss has colossal HP (600 for the lowest, 12500 for legendaries), and they often require more than one player (at least the weaker players, strong trainers can beat tiers 1-3 solo)
You have 3 (5 for legends) minutes tp beat him, and you can get knocked out as many times as you want without losing)
Once you beat it, you get a few random rewards and a chance to catch the Pokemon you just beat at a fixed level (20 without weather boost, 25 with the boost) with premier balls you get (note that it cant flee until you run out of them)
Raid bosses also have 5 different tiers, the first are the most basic, have 600HP, and you should be capable of beating them solo at around level 20
Tier 2 has 1800HP, and can be beat at around level 25-30, otherwise bring another trainer with you
Tier 3 has 3000HP, can be soloed between levels 30-35, and usually require 2-3 players if you have low levels
Tier 4 has 7500HP and even the best have to be in at least 3 (or 2, for Absol and Tyranitar) to beat it, bring more than 5 if you have low levels
Legendaries require 10 and more low levels, about 4-5 high-end trainers
Oh wow! Thank you. Such great information from everyone. If I have missed saying thank you to your reply, please forgive. I did read it and learned something from it.
If you have YouTube search for the Pokémon Go channel which has a few vids that explain the game & show you how it works (some info / game screens may be outdated now).
Click on the link to their (quick) explanation on Raid Battles which may also help.