We use this phrase to say or suggest that somebody continues to do something and doesn't change.
I tried going to a French restaurant last night but I didn't enjoy it.
That's a pity. I guess you should stick to fish and chips then.
Yes, no more foreign food for me.
I heard David Beckham is going to make a movie.
That's crazy. He should stick to what he's good at.
Yes, he should stick to modelling sunglasses.