What does a w at the end of sentences signify?


Sometimes I see people attaching a `w` at the end of sentences (or multiple
`w`s):

> なんか知らない間に人がめっちゃ増えてるｗ

This is in an informal context, usually while chatting. And sometimes, just:

> wwwww

What is that `w` doing there? How do I read it?

