A sweep is a situation in which one team wins all of the games in a series against a single opponent. While the term is used in other sports as well, it is of more common usage in baseball given that teams typically play against each other for two to five consecutive games before moving on to face another opponent.
The lowest level of sweep is the doubleheader sweep, when one team wins both ends of a twin bill. The next is the series sweep, when one team wins all of the games scheduled between two teams over a short span of days. The term is usually only used when there are three or more such games. Next is the season series sweep - much rarer - when one team wins all of its contests against one opponent during the season. This becomes significant when the two teams host each other for at least one series in each of the two ballparks.
Finally, the most significant level of sweep is the postseason series sweep. This means one team winning all three games of a Division Series, or all four games of a League Championship Series or of a World Series.
It is customary, when a team is likely to complete a sweep, for fans to bring brooms and other cleaning implements to the ballpark and start waving them prominently as the eventuality of the feat becomes more likely.