In television, the story is broken into parts or acts, which helps define the dramatic structure of the episode. In layperson's terms, the act breaks are where the commercials go.
While dramatically speaking it is true that stories can be said to have three acts — essentially a beginning, middle, and end — hour long television scripts actually tend to have four acts, with the divisions coming literally at the commercial breaks - roughly the quarter-hour, half-hour, and three-quarters-hour marks. This will usually be noted right in the shooting script this way, in contrast to the theatre convention of using Intermissions. A four-act hour-long drama script can also include The Teaser, The Tag, or both.
In Act 1, characters and general mood are established. Towards the end of Act 1, the source of antagonism is introduced and the main plot point unveiled. To use the Buffy episode "Hush" as an example, we meet the Gentlemen and they steal everyone's voices as Act 1 ends.
Act 2 involves the characters striving to overcome the problem that lies before them, generally starting off at a low point, working towards a seeming victory/high point halfway through, and then falling as Act 2 approaches an end. The end of Act 2 is the lowest point of the story, where the main characters are threatened with imminent doom. In "Hush", this is the bulk of the show, when no one can speak.
To clarify the point above, Act 2 itself generally has two halves separated by a peak, with the first half Rising Action towards a seeming victory, and the second half falling action towards a seeming defeat. If one splits Act 2 in half this way with, say, a commercial break, then Act 2 becomes Acts 2 and 3, and Act 3 becomes Act 4.
The last act is the shortest act, and wraps everything up when the hero(es) manage to pull off a victory at the very end in a final battle. Buffy manages to defeat the Gentlemen in a final pitched battle and everyone's voices are restored.