Re-enactment, the term itself, means re-living old/historic times. Some explain it as: 'Acting famous battlefields from the history books.' But re-enactment is actually not about battlefields at all, it's just that people prefer to re-enact battlefields. Re-enactment can be anything, world wars, Napoleon's time, middle ages, et cetera.
So, more basically put, people go together in a club, all take on the role of civillians, soldiers, et cetera. Dressing up like them and can experience what it was like to live in those times.