"Existencia de trabajo" - Can Someone Explain This?

Is there anyone from the Spain who can explain this feature that's available to workers in Spain.

I understand that it's like an unpaid sabbatical leave

Say for example one's parents were ill and you had to spend time with them, wanted to do some study or even if you wanted to move jobs, I understand that you can ask for this as a part of the constitutional rights within employment law. In essence I understand that the employer cannot turn down this request.

I understand there are times as well, you can ask for it for 2, 6 or 12 months.

Could someone please help with this for me?