Hella

Hella is a slang word that is a kind of short form of "hell of", and people use it to mean really or very when they want to intensify an adjective. The best way to understand this word is to look at it in usage:

You: It's hella hot.
Him: Yeah, why don´t we go to the beach?

When you say "hella hot" you mean that it's really hot.

Here's another example:

Him: This paella is hella good.
You: It really is. It was such a great idea to come to the beach today!

You're likely to hear young people use this expression, but since it's a compressed word, it may be difficult to recognize. But when you hear it, you'll know what it means!