Okay, first see preferred stock for the gist.
Preferred stock is preferred because stockholders with these puppies are paid before common stockholders if a company goes belly-up. If your stock says "preferred stock—ARPS," then the rate paid out depends on a specific set standard or yardstick (usually T-bills). So if T-bills (or whatever the benchmark the company uses) suddenly pay out more, your stock will, too.
And that's how it's adjustable.