What does the green wavy line under a word or phrase mean in Microsoft Word?

The green line is supposed to alert you on a grammatical error. Introduced in Microsoft Word 2003 the green line is a boon and curse of its own. Itstill hasn't been perfected and can annoy all of us to the highest.

Depending on which version of word you're working in, like 2016 suite for instance, that line is blue, not green.