In theater, an act (noun) is a short performance that is part of a longer program.

When part of a play or opera, each individual act is separated by curtain falls.

When part of a television program, each individual act is separated by commercials.

In film, each individual act is usually separated with a break in the action or plot development. Traditionally, this is accomplished with a change in music from whatever is standard for the film to something more light.

It should be pointed out that not every form of entertainment is broken up into acts, and even of those forms that regularly do, not every individual program follows the general rule.