A Saltwater pool is a pool that is warranted for use with a Saltwater Chlorine generator. Chlorine generators, also known as Saltwater systems, consistently make small amounts of chlorine when you run the system so you do not get the high and low chlorine levels that you would with shock and chlorine tablets or powder.
A saltwater system provides a more steady chlorine residual that would prevent algae, and it's much easier to maintain and less expensive to run than most traditional methods.
Here's a good article on saltwater pools which will help you learn more: