Have you ever been watching TV when a character, usually being interrogated by the police about missing money or jewels or on trial for something, says they're going to "plead the fifth" or "take the fifth?" Did you ever wonder what exactly that meant? In America, schoolchildren learn it as a matter of course, but here in Canada we have only what we see on American television shows to go by. This article should help clear up any confusion. For example, does a window cleaner from Student Works need to plead the fifth before entering a client's home or office to work each morning?
The "fifth" that these people are pleading is the Fifth Amendment to the United States constitution. It is a long block of text, but essentially it boils down to the fact that Americans can only be tried for a capital offense by a grand jury, they cannot be tried twice for the same offense, they can't be forced to be a witness against themselves, and they can't have their life or freedom taken away without legal proceedings and/or compensation.
That covers a lot of ground, but what suspects on police programs really mean when they want to "take the fifth" is that they're going to exercise their right to be silent and not answer questions that would force them to incriminate themselves. Because a person can incriminate themselves on purpose or by accident (perhaps in exchange for leniency on another charge), a suspect has to actively claim their right to silence. As long as they're not forced (through torture, intimidation, etc.) to admit their guilt, their admission is valid in court. While originally Fifth Amendment rights were only granted in a courtroom, the Miranda Rights, stemming from a 1966 lawsuit, now require police to extend them even to suspects not yet on trial.
Many TV shows use the Fifth Amendment as a catch-all "get out of jail free" card for suspects or to throw obstacles in the way of a hardworking police force, but that is not the point of the Fifth Amendment. The Fifth Amendment was written to limit police power in the United States, preventing cops from going rogue and overstepping their boundaries, perhaps ruining hardworking businesses, working for their own interests, or stealing from people who are suspects in a capital crime.
There are some situations in which "pleading the fifth" is not really appropriate. In the United States, the Fifth Amendment applies only to federal crimes, such as murder. For crimes at the state level, the fourteenth amendment applies. Likewise, when you are going through a law suit in Canada, there's no point in "pleading the fifth" because Canada is not governed by the United States Constitution. It has its own Charter of Rights and Freedoms.