What can my followers see?

If someone follows you on Foursquare they will only be able to see your publicly available information (just like Twitter, Instagram, etc).

That includes your name, your tips, if you've rated a place, and your tastes. There's no difference in what a follower and a non-follower will see.

Any information around real-time location (your Swarm check-ins, for instance) will never be shared with people who follow you on Foursquare.