A munch is a social gathering that usually takes place in a local pub, bar, restaurant or coffee shop. All munches are slightly different depending on the venue and the make-up of the people who attend, but in general the ethos is always the same in that it is an opportunity for members of the local kink community to get together to socialize in a low pressure, non-play environment. Think about going to a local book group or photography group social; a munch is remarkably similar to that except that the common interest is kink. Having said that, no two munches are exactly the same. That said, most of them are open to all types of kinksters and will attract a fairly diverse group of people. Some munches are themed, and cater toward under 35s, those interested in D/s relationships, or ladies only. What they all have in common is that they are not play events or opportunities to "pick up" partners, but are social events to meet new people and make friends.
Lots of munches have a "meet and greet" service for people who are new, where the organizers, or an established member of the group, will be there early to meet you at the venue and introduce you to people. This is a service worth signing up for as it can be very daunting meeting a new group of people. Knowing someone is expecting you and will help you settle in can make that a whole lot easier.
Oh, and my final tip is this: All munches are slightly different, so I really recommend that you try as many as you can find in your area to find the one that seems to fit you best.