What is it called when a prisoner is let out of jail by the president?

When a prisoner is let out of jail by the president, it is referred to as a "pardon." A pardon is an act of clemency that absolves the individual of their crime, effectively restoring their rights and removing or reducing the penalties associated with their conviction. In some cases, the president may also grant "commutations," which reduce the severity of a prison sentence without fully pardoning the individual.