In the Catholic faith, worshippers confess their sins to a priest who assigns penances for them to perform and grants them forgiveness for their sins. Here, the friar puns on the word “riddling.” To riddle can mean to present someone with a riddle (puzzle), or it can mean to pass something through a sieve to sift out the unwanted part. Friar Laurence is warning Romeo that if his confession is “riddling” (puzzling) then the absolution he will grant will be "riddling" (of little value).