Zero-day exploits refers to recently found vulnerabilities in a computer software program that has been in existence but was hitherto not known and addressed by the software security experts, however, these vulnerabilities were known to hackers. While the existence of these "loop-holes" in the software can go on unnoticed for several years, hackers can take advantage of it to cause harm to the computers' programs and data.
When these attacks occur, it is called a zero-day because the attack took place on the very day that the loop-hole was discovered in the software. So exploitation has already taken place before a fix is carried out.
