Ionic V3 vs Ionic V2

Can I keep working on my v2 ionic project and start a new v3 project? Is ionic v3 backward compatible?

Yes.

What does that mean? There is stuff in v3 that doesn’t work in v2, of course.

What about v2 in v3? Every stuff working in V2 works in V3 too?

Not the stuff that got changed, but I am not aware of anything that is causing to much problems.
Bigger changes where in switch from Ionic Native 2 to 3.