If you remember the VCR days, imagine your hard drive is a copy of Bambi. You, in preparation for a family event need a tape to store footage of the event on. You decided that you haven’t watched or wanted to watch Bambi in a long time so you designate that tape as the one you’re gonna use when the party day comes.

At this point your hard drive (the copy of Bambi) has been designated as useable space for new data to be written in the future.

Bambi is not lost yet and wont be until you write to that tape, therefore if you wanted to you could watch Bambi in the time between now and the party even though you plan to overwrite it. Once Bambi is overwritten, its no longer recoverable but the interim between now when you designate it as useable space and when the space is used, the data persists.