There are a ton of old posts on this you can search through, but I can give you a quick answer.

A 'mode' is an inversion of a scale. Yup, just like you can invert chords, you can invert scales. To get a mode, all you have to do is start playing any scale from a note other than the root. Example - Take a C major scale, and start playing from the G note (the 5th). You'll have a G mixolydian (one of my favortites)

Going from a major scale you get 7 modes. You can look these up on GT or on Wholenote.com. Each mode has a distinctive sound, or feel, to it, and is useful in different situations. The mixolydian that I mentioned is used in blues and celtic music. Phrygian sounds really dark and is used in just about every form of music (from flamenco to death metal). And etc . . .