When you normally delete a file, it doesn’t actually delete it to save time, it just marks the space as free, so any new files can be written into that part of your drive.

But the actual data just remains there until a new file is written to the storage.

SecureErase does the second part without making an actual file.

Normal delete:

File: 01010101 -> no file:01010101

Secure erase:

File: 01010101 -> no file:00000000