The key of a song is a reference to the notes and chords that are in it. For example, a song in C Major will primarily use the notes C D E F G A & B, and mainly use the chords C Major, D minor, E minor, F Major, G Major, A minor, and B Diminished. Of course, they don't have to only use those notes, but they're the most common ones seen.

If you don't know what major, minor, and diminished means, don't worry! There are plenty of resources available. I'd suggest starting with a free guide like this one, and if you still feel like you want more knowledge consider enrolling in a theory class at your local community college!

edit for spelling & grammar

What these two sexy stud muffins just said. ^^^

...yeah, exactly. Or more simplified: It's a bunch of notes that sound good together. Easiest of them is C Major (white keys). The others are (again, really simplified) essentially the same intervals (when relying on relative minor scales), but shifted around the kayboard, so some white keys get substituted by black keys.

Point of scales is, that if you manage to make song from chords, that are made from notes, that are in particular scale, it will have really high chance of nothing sounding dissonant. ...which is a good thing at times, bad at another. Depends on your taste.