Previous versions of OS X used to have a Secure Empty Trash feature, which would securely delete the contents of the Trash. What this did was overwrite the files with zeroes, making it much harder — nearly impossible, in fact — to recover the files.
Unfortunately, when OS X El Capitan was released, Apple removed the Secure Empty Trash feature. There are still a couple of ways you can securely delete files in El Capitan.
Read the rest of the article on The Mac Security Blog.