“Here. Take an extra one, just in case.”

Your nephew came over to visit. Now he's leaving to go back home. It's a long drive, so you gave him something to drink. Now you think that he might need another drink, so you hand him another one and say:

Here. Take an extra one, just in case.

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?

(do something) just in case (something happens)

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

In the example above, you gave your nephew two drinks because you thought he might get really thirsty. In other words, you gave him two drinks in case he got 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.