A judge can only accept testimony or other forms of evidence (like documents or photographs) in a trial if they are relevant to an issue the judge must decide. That’s why you may hear trial lawyers on t.v. or in B.C. courtrooms object to some evidence as irrelevant, and why the judge may rule the evidence cannot be admitted.
Courts have tried to explain relevance in statements like these:
Evidence is relevant “where it has some tendency as a matter of logic and human experience to make the proposition for which it is advanced more likely than that proposition would appear to be in the absence of that evidence.
Supreme Court of Canada quoting from the Law of Evidence (1996) by Paciocco and Steusser at p.19 in R. v J.-L.J.  S.C.J. No. 52 at para. 47
And a little more succinctly:
The threshold for relevance is not high. Evidence is relevant when it renders the existence or absence of a material fact in issue more or less likely.
B.C. Supreme Court in R. v. Tremblay, 2012 BCSC 2105 at para. 22
Evidence is relevant when it tends to increase [or decrease] the likelihood of the existence of a material fact at issue in the proceedings.
Ontario Superior Court of Justice in R. v. M.C., 2012 ONSC 868 at para. 11
Relevance is really a matter of everyday experience and common sense. Information can only be admitted as evidence if it helps to prove or disprove a fact that matters in the particular case. So if you are participating in a trial and want to testify about something or have another witness testify about something, you have to be able to answer this question:
"To what material fact at issue in the trial is the evidence logically connected?"
Consider some examples that judges often deal with – first, evidence that’s not relevant. In family court hearings separated parents may want to tell the judge about each other’s faults. Buts. 37(4) of the B.C. Family Law Act says that when deciding issues involving a child’s best interests a judge may only consider a person's conduct if it substantially affects one of several factors identified in the Act. This means that parents can’t testify about each other’s bad conduct unless it affects one of those factors. If it doesn’t affect a factor, it is irrelevant.
Next, evidence that is relevant. At a Small Claims Court settlement conference you may be ordered to exchange copies of “all relevant evidence”. That means you must give the other party all the material you possess that is connected to the issues in the case – not just material that helps your case, but also material that may hurt it. It’s all relevant.
Confining evidence in a trial to what is relevant helps to ensure that trials finish within the time allotted and avoids arguments about red herrings that distract the parties from the real issues. Lawyers can help their clients by explaining in advance what evidence is irrelevant and why. And people going to court without lawyers can improve their evidence by thinking in advance about what evidence is logically connected to the issues the judge will have to decide, and what isn’t.
To see the kind of facts that may be relevant to various family court issues, see Guide to preparing for a family court trial in Provincial Court.