Scansion is the meter or rhythm of Western poetry, usually called 'scan'. The scansion values are based on measures like beats in music, with the 'bar' created by the basic rhythm. A simple rhythm like 123456 is used as a working measure for the rest of the poem, which is supposed to be in similar measures. That's not necessarily a rule in Oriental, haiku, Arabic, free form or some types of modern poetry, but it's the common rule otherwise.
Examples of Scansion:
Complex scansion 1234-567- 8910/1234-567-8910|
Incorrect scansion unless qualified by form: 123456/1234
Free form or open scansion: 1234/12345/123456/123