Yikes is what you say out loud when something suddenly shocks you, scares you a little, or makes you think "oh no, that is bad". It is a whole sentence by itself, said with feeling: "Yikes!" Two things make it different from a hard word like "help". First, it is light. You use it for the price of a repair, a huge exam score you did not get, a friend's terrible haircut, a spider on the wall — real fear uses other words. Second, it is friendly and slightly childish, so it is safe everywhere: children say it, teachers say it, it appears in comics and cartoons. It is much more American than British, though British speakers know it well. Younger people online have stretched it in a newer way: writing "yikes" under someone's post means "that was an embarrassing thing to say and I am quietly judging you" — not fear at all, closer to a wince. You will also see it doubled for that meaning, "yikes yikes", or written as "big yikes". Grammatically it stands alone; you cannot say "I yikes" or "very yikes" in careful English, although people do play with it on purpose online. It can start a sentence: "Yikes, is that the time?"