A rut is a groove in the earth, like an indent caused by wheels. It's also a boring routine that can be hard to break. If you’re stuck in a rut, you’ve been doing the same old thing for too long.
If you see a groove in the ground, especially one caused by a bike or car, that's a rut. If a rut is deep enough, you could get stuck in it, which may be why the word has another meaning. If your life has become boring, stale, and monotonous, you're in a rut. Someone who works the same job for decades may feel like they're in a rut and need to make a change.