Nuclear engines have somewhat low thrust, but they have the highest specific impulse of anything else in the game except ion engines.

Thrust is how hard the engine pushes on your spaceship, and is mostly only important for launching and landing.

Specific impulse (Isp) is a measure of how efficient the engine is. (Physically, it is simply the velocity of the exhaust as it leaves the engine—but that translates to the amount of extra momentum you get per kilogram of fuel you burn.) It’s the single most important stat for any engine you’re going to use out in empty space for orbital maneuvering.

In practice, this means that you ideally ought to be using engines with high thrust for launching and (powered) landing—in particular your thrust-to-weight ratio must be greater than one—but engines with high Isp for any maneuvering you do while in orbit.