See call protection. It's just the language that spells out if or how a bond is callable by the issuer. Let's say Nike wants to buy a bunch of grammar schools in India and it doesn't have the cash on its books to do so. It would issue debt to the public, but current interest rates are high and Nike thinks that it will have a lot of cash coming in over the next few years as its shoes will get much cheaper to make after this acquisition. Consequently, Nike offers the bonds at a high rate - 8% - but there is a call provision. That is, they can buy the bonds back at 102.5 cents on the dollar at any time after the first two years have passed. The investor makes a little premium if Nike does redeem and Nike is happy because they have less debt outstanding (and smaller interest payments).