, I need to understand it what happens when I press "Enqueue" In web UI and set it to default queue

The Task ID is pushed into the execution queue (from the UI / backend that is it), Then you have clearml-agent running on Your machine, the agent listens on queue/s and pulls jobs from queue.
It will pull the Task ID from the queue, setup the environment according to the Task (i.e. either inside a docker container or in a new virtual-env), clone the code/apply uncommitted changes install the python packages etc. then it will spin the code which will use the configuration in the UI (instead of logging into the UI, when executed manually)
Make sense ?