Here.

Say "here" when you're giving something to someone:

Here. Take this.

an extra (something)

"An extra ___" means one more than you need of something:

Do you have an extra pen?

There's an extra towel in the closet on the top shelf, if you need one.

(do something) just in case (something happens)

Doing something "in case" means that you do it to prepare for a possible result.

For example, if you give someone an extra drink "in case", it's because you think they might get really thirsty. 

So you can say "(do something) in case (something happens) like this:

Here. Write down the phone number in case you can't find it.

Or you can say "just in case". To do something "just in case" means that you're doing it even though you don't think it's needed. You're doing it to be extra careful:

I'd better lock it up just in case.