Microsoft Word marks text with a green wavy line when the program detects a grammatical error in your document. There is an option in the program to "check grammar as you type." You can switch it on or off as to your preference. You have to right click the green wavy line to display the suggested corrections.
You will usually see it under a few words as part of the whole of the sentence. It could be mistakes involving a subject/verb agreement. This feature is beneficial to all of the people who use it.