A munch is an informal meeting of people interested in BDSM fetish, an opportunity to meet like-minded people, talk, find out about good fetish shops or clubs. For many, it's their first foray into the scene. Most munches take place in pubs or similar ordinary venues. They're almost always "vanilla", ordinary clothes and no play. The term munch is also used by the swinging community so if unsure of what type of munch is being promoted, it is worth confirming.
There are also other meetings called sloshes, which are similar, only the focus is less likely to be on a meal. The word "slosh," refers to the liquid refreshments, not to the state of the attendees. If you prefer not to drink, no one will think it odd in any gathering if you stick to soft drinks or coffee. You won't be the only one.
What you can expect to happen at a munch is not really much different than what might happen at a company party or a class reunion or a bowling banquet. If you accidentally wandered into the room, what you'd see is a group of people, numbering from 10 or 12 up to 50 or 60, depending on the location and popularity of that munch. Munches are almost always held in public locations, so fetishwear isn't appropriate. You might note a few more pieces of leather clothing - vests, skirts, jeans - but not of a style to cause comment and nothing to tip off anyone.
Munches are designed to provide a safe and public location for people with a common interest to gather. Not every munch or every munch organizer has exactly the same agenda, so bear in mind these are general truths. It's a good idea to let the host know you're coming to a munch if you've never been before, just to make sure that your information is current or that you don't need an invitation or a reference from someone. In addition, some locations make it necessary to know how many people to expect. Most munches are open to the public, but rules do vary, so it's wise to check. The host will appreciate knowing to expect a new face, as well.
The usual age range in most groups is late 20's to late 40's. That's not to say that you won't see both younger and older people there, simply that the bulk of people would fall into those ranges. Munches are usually a good mix of males and females, of singles and couples and of Tops and bottoms. That is not to say that it's evenly divided among those categories, only that it's a mix of them. It seems that there are more submissives than dominants, but the ratio is not usually wildly skewed, either.
What shouldn't you assume a munch is, unless you're told otherwise specifically? It's not a dating service. It's not a singles group. It's not a swinger's club. The best reason to attend a munch is to find people who have similar interests as you, people with whom you can talk and learn from, whether you are a novice or an experienced player. The more people you know, the more likely it is that you will find someone who captures your fancy.
If you feel uncomfortable with a situation, please tell the host. We would very much rather deal with something at the time it happens than find out later that there was a problem we never knew about. Bear in mind, too, that these ARE public events. While it's probably a good start that the person you've met who seems so interesting is attending a munch, it does not vouch for his or her character. Don't assume that everyone you meet there is safe and trustworthy simply due to their presence. Most are, in fact, but you still need to behave as responsibly as you would had you met them in other circumstances. If you do run into someone you'd like to get to know better, giving your email address is usually safer than giving your phone number.
Bear in mind that as long as you keep servers happy and don't antagonize other patrons, restaurants will usually welcome you. If you're in a public room, the people around you did not consent to hear your overly-loud descriptions of the exquisite spanking you delivered last night. They have a right to their privacy and the enjoyment of their meal as much as you do. However, I think conversations held in normal tones of voice are the business of those having them, and if someone chooses to listen, they should be able to deal with what they hear. If you have a more private location, those concerns will be lessened. Take your cue from those around you who have been there before.