Protection Against Self-Incrimination
Section 13: A witness who testifies in any proceedings has the right not to have any incriminating evidence so given used to incriminate that witness in any other proceedings, except in a prosecution for perjury or for the giving of contradictory evidence.
Section 13 protects a person from being compelled to incriminate themselves. In criminal cases, an accused is not required to take the stand in their own defence. In other words, they are not compellable -- they have the right to remain silent. By contrast, witnesses other than an accused can be (and often are) compelled to appear in Court and give evidence. Section 13 of the Charter protects these witnesses who are compelled to give evidence. It ensures that these witnesses cannot have that any compelled incriminating evidence used against them in later proceedings. In other words, it is a bargain struck between the Crown and a witness being forced to give evidence – the Crown can compel the witness to answer incriminating questions, but in exchange, they cannot use that evidence to incriminate the witness in subsequent proceedings (R v Nedelcu, 2012 SCC 59, at para 79 [Nedelcu])
There are two important factors that must exist to “trigger” section 13:
Compelled Evidence: Section 13 will only protect witnesses if they are being compelled to give testimony, or if they are compellable witnesses. If a non-compellable witness testifies voluntarily, their evidence can be used against them in a later proceeding (Nedelcu at paras 102, 103 quoting R v Henry, 2005 SCC at para 34).
Incriminating Evidence: Section 13 only applies to incriminating evidence, meaning evidence that may lead a judge or jury to infer that a person is guilty of the act they are accused of (Nedelcu at para 29). Even if the evidence seemed harmless at the time it was given, it can become “incriminating” if at a later date it becomes relevant in proving that the witness committed a crime. Evidence that is not incriminating is not protected by s. 13. For example, evidence may not demonstrate that the witness committed a criminal act, but nonetheless it may show that the witness has a horrible memory or has changed their story (in other words, demonstrating they have bad credibility). This evidence is not incriminating – it can be used in later proceedings (Nedelcu at para 28).
Exceptions to the Rule
Section 13 applies “except in a prosecution for perjury or for the giving of contradictory evidence”. Perjury and giving contradictory evidence are crimes that are committed when lying under oath, or by give false evidence/testimony.
This rationale for this exception is best explained by reference to the purpose of this section. Section 13 is a bargain – it permits the Crown to force a witness to give evidence that might be incriminating, but in exchange for full and frank evidence, the Crown cannot use it against that witness in other proceedings. It exists to assist the administration of justice – witnesses are more likely to tell the truth when they know that evidence cannot be used against them later. The perjury/false evidence exemption exists because the witness did not live up to their end of the bargain – they did not provide full and frank evidence. If they lied on the stand, the Crown is permitted to pursue them for those lies.
Section 13 vs the Fifth Amendment of the United States Constitution
Section 13 is often explained by reference to its more well-known American counterpart – the Fifth Amendment. In the United States, the Fifth Amendment permits a witness to refuse to answer any question that may incriminate them (a.k.a. “taking the fifth” or “pleading the fifth”).
This is not how the law works in Canada. In Canada, a witness can be forced to answer incriminating questions. As part of the bargain, however, the Crown cannot use that evidence to incriminate the witness in another proceeding.
Section 13 is a continually developing Charter section as Courts grapple distinctions that are easy to state but hard to implement in practice (especially the incriminating vs non-incriminating divide). For further reading on the development of s. 13, see R v Kuldip,  3 S.C.R. 618; R v Noël, 2002 SCC 67; R v Henry, 2005 SCC 76 and Nedelcu to get an understanding of how s. 13 has developed over the past 25 years.