From LQWiki

/dev/zero is the file system name of a device that constantly delivers zeroes. It is complementary to /dev/null that receives data without processing them.

Examples:

dd if=/dev/zero of=/tmp/file

Fills /tmp/file with zeroes coming from /dev/zero.

dd if=/tmp/file of=/dev/null

Reads /tmp/file and "throws the data away" by sending it to the null device