My experience is that poster sessions at conferences are great catalyst for starting conversations about a topic you know much about, with many different people. Most of the people you will find you are educating them about your topic and giving a summary of your findings and answering general questions. Then there may be the odd few very knowledgeable professors or experts in the field (on your particular topic) who may ask you much more specific questions and a much richer conversation develops where you will have new insights about your research, think of new methods to test for something you never considered before, and develop new ideas you would never have before.
Then during the time slots you are not presenting your poster, you get to walk around and see all others works and ask them questions. Hopefully you can find some posters about topics similar to your topic so you will be able to get into some good conversation. Generally, I never found anyone to be critical of my poster content, even some where I knew later that certain things were not presented in the best ways, and the criticism would mainly be confined to something like "Did you try doing it that way? That's how I would have approached that". So as mentioned by Babcock_Hall, there is nothing really to be worried about. I think oral presenations at the conferences are much more stressful!!