Well, they are trying to say the comic humor depends on the situation that things are in. Ordinarily, a boss getting angry may not be funny, but if the boss is getting angry because of a situation that we can see is ridiculous (but the boss himself cannot see it... so he is angry), then it becomes funny.
The term "situation comedy" (which has been defined very well above) is used to distinguish from other forms of comedy, such as "slapstick" (like The Three Stooges) and "sketch" comedy, which involves short vignettes like those seen on Saturday Night Live.
Since the Parser mentioned I Love Lucy, I thought I'd post my favorite scene in a classic episode (which is also very appropriate for an ESL forum). The Ricardos are visiting Paris for the first time, and Lucy naively exchanged currency with a con man, not knowing that he gave her counterfeit French francs. When she was arrested, she felt helpless because she didn't speak French. At the police station, the cops have just arrested a German man for being drunk in public; the German doesn't speak French, either, but he speaks Spanish. When Lucy's Cuban-born husband arrives, a "translation" line is arranged.