In December, Google said that they would release ARCore 1.0 in the coming months, they have had some time to work on it now and it should quite soon be ready for release.
AR+ is an upgrade of the original AR in Pokémon GO which makes catching Pokémon much more realistic.
They now know that you are there, and if you walk too fast against it it will flee.
You now have to find the Pokémon in grass.
Conclusion: AR+ is a great addition to Pokémon GO, but they could have waited until ARCore 1.0 comes out so Android could have gotten it the same time as iOS.