Hearsay is an out-of-court statement tendered for the truth of its contents.
While all relevant evidence is generally admissible, hearsay is presumptively inadmissible, given the dangers it presents. Because hearsay is declared outside of court, it is often difficult for the trier of fact to assess whether it is trustworthy.
Hearsay dangers arise notably due to the absence of contemporaneous cross-examination of the hearsay declarant before the trier of fact.
R. v. Khelawon, 2006 SCC 57 (CanLII), at paras. 35.
The core hearsay dangers are as follows—
The declarant may have misperceived the facts to which the hearsay statement relates;
Even if correctly perceived, the relevant facts may have been wrongly remembered;
The declarant may have narrated the relevant facts in an unintentionally misleading manner;
The declarant may have knowingly made a false assertion.
R. v. Bradshaw, 2017 SCC 35 (CanLII), at para. 20;
R. v. Khelawon, 2006 SCC 57 (CanLII), at para. 2;
R. v. Baldree, 2013 SCC 35 (CanLII),  2 S.C.R. 520.