When you delete a file normally, Windows only removes the pointer to the data. The actual bytes remain on disk until overwritten by something else — and can be recovered with widely available tools.
Traditional overwriting (writing zeros or random data over the file) works well on HDDs but does not reliably work on SSDs. SSDs use wear levelling and spare areas that redirect writes, so overwritten data may still exist in another block.
Secure File Shredder addresses this by using encryption on SSDs: even if file data remains somewhere on the SSD, it is designed to be unreadable without the destroyed key.