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What is  data overwriting?

Simply, data is stored as magnetic bits in the disk and will occupy specific locations in the disk as directed by the operating system.  When you delete the data,  it will normally be marked as deleted, but the data content is still there so recovery is always possible. The data content will stay there until some new data replace it partially or completely, resulting in data overwriting.
Simply, it can be illustrated this way. Imagine  a word "OLD DATA" is stored in disk as data. Assume the first number "1" is the system bit that tells the operating system that the space is reserved for some existing data that is currently live and active.
Figure to show old data
Diagram to show data deletion
Picture to show data overwriting
Because this is existing data, it can be read normally at any time  and the data space is always locked so no other data can come and occupy the  space here.
Now you perform a deletion. What the operating system does is it will change the system bit from "1" to "0". With that the data " OLD DATA" is marked deleted and you now will not be able to read it through the operating system in a normal way. This data space is also made available for future storage of new data should there be a need.
Of course if you can find a way (for example, by using some data  recovery software) to turn the system bit back to "1"  or simply scoop up all the deleted data with system bit set to "0", you can recover the lost data easily. No sweat.
However, before you can recover the deleted data, let say you write a new piece of data "NEW FILE".  The operating system scan the disk and find that this data space that marked "0" is available for us. It will write the new data "NEW FILE" into this data space and at the same time update the system bit from "0" to "1" to complete the ownership. Now  we have a new master in the house !

In short, "OLD DATA" is now completely replacced with "NEW FILE". Obviously you can do nothing to recover the earlier data it has been "overwritten" and gone for good.