new to this - what is google sandbox?

It's the theory that brand new sites get held back in google for the first few months no matter what you do. Google has said there's no sandbox filter as such. What people notice is real though - a new domain has no links and no history, so google doesn't know how far to trust it yet and it ranks slowly at first. Same practical result either way: expect a slow start and go after easy long-tail terms early.