It can be a tricky difference to fully understand, but put simply, your overall fitness is a collection of measurements of your physical qualities, and conditioning is your ability to use those qualities to adapt and perform in a given environment.
To go into a bit more details, your fitness can be measured with things such as your resting heart rate, or Vo2 max, which suggest how well you’ll be able to perform certain physical tasks.
Conditioning on the other hand doesn’t really have measurements, as it’s more directly related to the physical and mental stamina required to use those physical qualities like your resting heart rate, or Vo2 max, to their full potential. It’s your ability to push through and keep going.