Final answer:
A green wavy line under a sentence indicates a grammar or spelling error.
Step-by-step explanation:
A green wavy line under a sentence indicates a grammar or spelling error according to most word processing software. It serves as a signal that there may be a mistake in the sentence that needs to be corrected. For example, if there is a misspelled word or a grammatical error, the green wavy line will appear to alert the writer.