A callable bond is a bond that can be redeemed by the issuer prior to its maturity. The first call price is typically par plus half the coupon, which gradually reduces over the life of the bond, although there has been a shift in this convention, with issuers sometimes demanding a shorter non-callable period (giving them more flexibility), but compensating the investor by increasing the first call price to par plus 75% of the first coupon.
If yields in the market have fallen substantially, there may be an incentive for the issuer to call the bond and refinance at a lower rate. This creates reinvestment risk for the investor so the coupon on a bond with a call option is normally higher than a non-callable bond. Callable bonds are a common feature of both markets so yield to worst (yield taking into account call options) is the preferred yield when looking at high yield bonds in both markets.