This fancy term simply refers to the process of analyzing a poem's meter. When you practice scansion—when you scan—you read a line of poetry, counting its feet, finding the stresses, all in the name of sussing out which meter it follows (if any).
Here at Shmoop, we scan poems all the time, and we mark our stresses with both bold and italic font.
So, for example, if we were going to scan the first line of Twelfth Night, it might look a little something like this:
If music be the food of love, play on.
This line contains an unstressed syllable (in normal font), followed by a stressed syllable (in bold, italicized font), and that pattern gets repeated five times. Boom. We have iambic pentameter. Nifty, right?
Of course things get really interesting when lines become trickier to scan. That happens when poets deviate from the meter they've chosen. Why would they do such a thing? To shake things up, to keep us on our toes, and to play off the meaning of the poems. The great poets never wrote in perfect meter, so be prepared for more than a few missteps as you start to practice scansion in your reading.
Speaking of practice, For Better for Verse is a great website that lets you bone up on scansion on a number of famous poems. Have at it, Shmoopers.