To scan a verse is to determine its meter — that is, to go through each foot and indicate whether each syllable is stressed or unstressed. The resultant pattern is called its scansion. We sometimes say a verse "doesn't scan," meaning the pattern of stressed and unstressed syllables is irregular — although it's wise to censure only verse that's supposed to be regular, since free verse isn't meant to have a fixed pattern of stressed and unstressed syllables.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.