What can someone do with my API credentials?

[TtheCreator]

I'm looking into using a putting my content on a external content selling/hosting website. (www.bitrunade.com if you want to know)

Before doing that though, I wanted to make sure this was a good idea.

The thing is that this payment solution requires me to give my paypal api username, password and signature.

My question is: What could someone potentially do with this information? (Like if someone wanted to misuse it or something got exploited or leaked)

I did actually try to look in the manual but that's a lot of information and I find it confusing to see what api is part of what and which actually use those specific credentials. That's why I thought I'd ask here if someone has a global idea that I can base a decision on.

Thanks in advance.

[Quandary]

PayPal provides a way for other applications, shopping carts, reporting systems and your website to talk to PayPal programmatically.   In computer programming, an application programming interface (API) is a set of subroutine definitions, protocols, and tools for building application software.  If someone had that info, they could not actually login to your account like you can - it's basically a way to pass info between your account and a 3rd party shopping cart  or website for example.   The API can do things – like check your PayPal balance, look up transaction details, issue refunds etc.