Scansion is the process of figuring out which syllables should be stressed when you read a poem. If you practice scansion in English class, you'll learn to determine a poem's meter based on the patterns of these syllables.
Scansion is a fancy literary term that simply means discovering the meter (or underlying structure) of a poem by marking where the stresses naturally fall. If you're using scansion to analyze verse, you can say you're scanning the poem. Formally, scansion involves making special marks used above both stressed and unstressed syllables, allowing you to easily see repeated patterns. Since the 1670s, scansion has meant "marking verse in metric feet," from a Latin root meaning "to climb."