A click is straightforward: it’s someone clicking on your ad (or tapping if they’re on mobile). Not all clicks are equal though:
Link clicks include any click on a link in your ad, even some internal Meta destinations. Outbound clicks specifically mean someone left Meta and went to your site. Clicks (all) refer to any and all clicks, including opening the comment section or liking the post.
Link and outbound clicks are important for conversions, but clicks (all) is also helpful. People liking and commenting on your ads generally helps with performance by signaling good health to Meta, and these are both counted by clicks (all).