Adding new words to the built in dictionary
Word has a great many words in its dictionary, and you can add to it any time you need to.
This is especially important in this day and age when new words and phrases are coined all the time in common parlance, but Word does not automatically update its files.
When you get the red error lines beneath a word, just right-click and hit “Add to dictionary”. Alternatively, you can add words to the dictionary when you hit F7 and go through the full error and spellcheck.
Adding a word to the dictionary will remove red error lines from all instances of the word in the future.
Look at a word carefully before you add it to the dictionary - if it’s misspelled when you add it, Word will consider the misspelling legitimate, which could lead to many more misspellings later on down the line.