Disk Utility > Erase Free Space does just that, it erases the available space on hard drives to make sure your deleted data is scrubbed off the hard drive.
When you delete via Empty Trash, the data remains on hard drives, just it's space is available for new data to go there, thus could be recovered using special software.
So the Erase Free Space is a Security Option to make sure that data is also removed, since it doesn't know where deleted data is, it just does all the free space with no data. It takes some time to complete.
If you use Secure Empty Trash, that will "scrub" the data off the hard drive as it deletes it, it takes less time than the Erase Free Space as it's only doing the area where your deleted files once were.
Can somebody explain how can I recover the speace I "freed"
If you reboot your Mac, it will learn the new drive space and that space is now free.
Check it under Activity Monitor > Disk Usage
in order to stop my Mac to keep recovering delted files?
Your Mac shouldn't be recovering deleted files on it's own, unless you installed some sort of software.
Once you Empty or Secure Empty Trash, those files in the Trash can are pretty much gone permanently.
I am just afraid that I used up all my free space and now I wont be able to save anything else anymore.
That is true, why you open Activity Monitor in your Utilities folder and click on Drive Usage.
You need to stay under 80% of the drive filled.
If you need more "space" you need to learn about storage drives to store your data pernamently, TimeMachine or clone backups don't perform pernament storage, they are backup systems of the boot drive.