Prior consistent statements are declarations made by witnesses before they take the stand that are consistent with the testimony they give while on the stand.
David M. Paciocco, “The Perils and Potential of Prior Consistent Statements: Let’s Get It Right” (2013) 17 Can. Crim. L.R. 181, at p. 181.
Prior consistent statements are generally inadmissible. Traditionally, they have been treated as inadmissible because they are out-of-court statements made in the absence of trial safeguards such as cross-examination and the taking of an oath or affirmation to tell the truth. The hearsay rule precludes the admission of prior consistent statements for the truth of their contents. Additionally, prior consistent statements lack probative value.
See R. v. Stirling, 2008 SCC 10 (CanLI), [2008] 1 S.C.R. 10, at para. 5; R. v. Dinardo, 2008 SCC 24 (CanLII), [2008] 1 S.C.R. 788, at para. 36
Put differently, repetition of a statement by the same person does not render it more likely to be true or corroborative. The repetition is self-serving and the source lacks independence. Lastly, given that the evidence will have already been adduced at trial through oral testimony, exclusion of prior consistent statements serves the desirable objective of trial efficiency.
         
          R. v. D.B., 2013 ONCA 578 (CanLII).
Exceptions to the rule against prior consistent statements
There are some exceptions to the rule that prior consistent statements are inadmissible, including narrative and recent fabrication.  The exceptions exist because the purpose behind exclusion is not served. 
See also blog post O’Connell, Edgar Statements: an Exception to the Rule Against Prior Consistent Statements, Dec 09, 2016.
As narrative, a statement is admitted as evidence of the fact that something was said or heard, but not tendered for the truth of what was said or heard.
Dinardo, at para. 37, R. v. J.A.T., 2012 ONCA 177 (CanLII), 288 C.C.C. (3d) 1, at para. 99; R. v. Magloir, 2003 NSCA 74 (CanLII), 178 C.C.C. (3d) 310, at para. 23.
Recent Fabrication
An allegation of recent fabrication is an allegation that goes to the heart of a witness’s credibility: an allegation that the complainant has made up a false story to meet the exigencies of the case.
 To be “recent”, a fabrication need only have been made after the event testified about.
 R. v. Ellard, 2009 SCC 27 (CanLII), at para. 33; R. v. O’Connor (1995), 1995 CanLII 255 (ON CA), 25 O.R. (3d) 19 (C.A.), leave to appeal to S.C.C. refused, [1995] S.C.C.A. No. 460.
It is not sufficient to invoke the principles relating to recent fabrication that there only be an allegation of fabrication. It must be recent fabrication in order to permit the introduction of a prior consistent statement.
R. v. Pangilinan (1987), 1987 CanLII 2432 (BC CA), 39 C.C.C.(3d) 284 (B.C.C.A).
The fact that the whole story of a witness is challenged does not, by itself, constitute an allegation of recent fabrication.
R. v. Campbell, 1977 CanLII 1191 (ON CA).
Recent fabrication is a rebuttal rule.  A prior consistent statement that is admissible to rebut an allegation of recent fabrication is not admitted to prove the truth of its contents. Rather, it neutralizes the challenge or allegation of recent fabrication. The evidence of the prior consistent statement is used to establish that the challenge is in error, not to show that exigencies the statement is true or that the witness is likely telling the truth because they said the same thing before.
R. v. D. B., at para. 37; Paciocco, p. 191.