Could someone explain exactly what intervals are?

I know it's a series of harder efforts, punctuated by light effort, but what's a typical pattern?

And, blueskies....I imagine the fat melting off my legs when I'm riding or working out. I can't do the calisthenics we do without relating it to an actual movement I would do in real life if I had the opportunity. "putting a box on a shelf...putting a box on a shelf..." is the kind of thing that goes through my mind.

I guess an active imagination would be helpful to stay motivated.

Karen