A nuclear thermal rocket (abbreviated to NTR) is a type of rocket engine which heats a propellant (usually liquid hydrogen) using a nuclear reactor. This is different from chemical rockets, which burn their propellant to heat it. NTRs have higher exhaust velocities than chemical rockets, high enough to triple a spacecraft's payload capacity (the amount of cargo or astronauts they can carry to their destination).
NTRs have the potential to be very fuel efficient while still providing good thrust, but their thrust-to-weight ratio is very small. A rocket's thrust-to-weight ratio (TWR) is the proportion between the amount of thrust the spacecraft can make and how much the spacecraft weighs. For example, a spacecraft that weighs 100 pounds and can produce 10 pounds of thrust would have a TWR of 1:10. An average chemical rocket has a TWR of about 70:1, but an average NTR has a TWR of 7:1. This means that NTRs have to burn for a long time to get the same acceleration as a chemical rocket. This is balanced by their high fuel efficiency.
NTRs have not been tested in space yet, but a special program named DRACO is going to attempt to do so.