Caveat. I would call it what it says it is, a measure of your training load. Not a measure of fitness.

Training load can correlate with fitness. Or it can inversely correlate with fitness. I only bring that up because the 3-4 peaks on my ~10 yeas of CTL are all when I was absolutely shattered and had to take months off. So as you scale the CTL peak, do so gradually, and start to tip-toe the closer to the top you get. It's always better, in my opinion, to be slightly undertrained (vs. some mythical optimal fitness) vs. overtrained.

To measure fitness either use race results or some repeatable test, like an FTP or ramp test.