Using Word's suggestions to correct spelling mistakes
Red squiggly lines underneath a word indicate that Word does not recognize the word, and therefore may be spelled incorrectly. Thankfully, there are several features available to help you with spelling, even if you do not necessarily know how to actually spell the word.
Word has a large built-in vocabulary. Right-clicking a misspelled word and using the contextual menu may give you some options for correcting it. You can also do an entire document, one word at a time, by hitting F7. Word will then go through each spelling and grammatical error with you.
Keep in mind that Word does not pick up all errors, especially homophones (words that sound the same but have a different spelling and meaning, such as their/there/they’re).
It is always recommended to carefully proofread your pieces, or have a friend do it if you need a fresh pair of eyes or don’t have much confidence in your own editing skills.