Okay, so imagine if you went to a big party and you saw somebody do something really, really bad - like steal all the cake! You might tell the host of the party that they shouldn't let that person come to their parties anymore.
Well, in medieval times, kings and lords would sometimes do the same thing, but on a bigger scale. They might decide that somebody had done something really, really bad, like break a big law or not follow the rules of their land, and they would tell everybody in their kingdom that that person wasn't allowed to come there anymore.
That was called a "ban". It meant that nobody was allowed to talk to that person, trade with them, or invite them to parties or events. They were basically kicked out of the kingdom and had to find somewhere else to live.
It was a really serious thing back then, and could ruin somebody's entire life!