When you call tune.run you are asking tune to run some trials of something. In this case you ares asking tune to use rllib to train an rl problem. It looks like you only have 1 trial.
You have only asked tune to train one thing. You could have done multiple by asking it to for example train with two different learning rates to compare performance or you could have set num_samples>1. Then it would run several different identical configurations so you could look at variability.
In those cases you have multiple trials some of which might be running at the same time and others which might be queued until others finish. For example you ask for 10 trials and you only have 8cpusand use 1 cpu per trial. You would have to wait for 2 trials to finish before 9 and 10 start.
The tune value you are looking at is telling you if the trial is still training or if it has finished.