I've presented posters at two larger conferences as an undergrad, AAPA and BARFAA. Mostly people just come around and rarely ask questions. Honestly, most people don't really care what your poster is on, but you will have a small few that will be interested. Just make sure you know what you are talking about because some people will ask you to explain your poster. Try to have a short summary of your poster. I would always ask if they had any questions after I explained my poster to them, most don't. If someone asks you something you don't know or didn't focus on, reply "oh, I didn't focus on that, but that sounds like something I could look into" or something along those lines. What conference are you presenting at?
Overall, I tried to use this time to network and visit other people's posters, looking for certain school I wanted to attend and asking about the schools.
Basically, don't sweat it. It is easy and the presenting time frame will go by fast.