Full Idea
When a variable is not combined with a quantifier (and so is 'free'), the result is, intuitively, semantically incomplete, and incapable of truth or falsity.
Gist of Idea
When a variable is 'free' of the quantifier, the result seems incapable of truth or falsity
Source
Theodore Sider (Logic for Philosophy [2010], 4.2)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.92