"Constantly" means "all the time" or "again and again many times". You use it like this:

She's constantly criticizing me. It's annoying.

The word "constantly" has a slightly negative sound. Even if you're talking about something that is usually positive, if you use "constantly" it seems like you might be a little annoyed.

People are constantly coming up to me and telling me, "Oh, you're so beautiful" but I'm like "Whatever."