You know how Firefox underlines misspelled words? On Windows you can right click the word and be presented with possible corrections, but on Macs you can't right click so what do you do when a word is underlined in red?
If you're actually stuck with an ancient mouse or trackpad that doesn't let you right-click, you can hold down Control while clicking to do the same as a right-click.
But for many years now, Macs have come with Mighty Mice and Magic Mice, Magic Trackpads and equivalent built-in trackpads on MacBooks that all let you right-click (two-finger click on the trackpads). So be sure to check your Mouse or Trackpad preference pane in System Preferences to make sure you've enabled right-clicking (or two-finger clicking) if you can. Don't just assume you can't because you don't see a separate right-click button on your mouse or trackpad; all the mice and trackpads I mentioned above have right-click ability; the mice have hidden sensors that can tell which side you pressed down on when you clicked.