Mostly it refers to the lack of authority of a judge to rehear a case after it has rendered judgment; they are "void of office".
In Chandler v Alberta Association of Architects 1989 2 SCR 848 (published at canlii.org/en/ca/scc/doc/1989/1989canlii41/1989canlii41.html), judge Sopinka wrote in relation to the "principle of functus officio":
"The general rule (is) that a final decision of a court cannot be reopened.... The rule applied only after the formal judgment had been drawn up, issued and entered, and was subject to two exceptions: where there had been a slip in drawing it up, and where there was an error in expressing the manifest intention of the court."