Zero day exploit is taking advantage of security vulnerability before it is known and fixed. Hackers are becoming faster at exploiting vulnerabilities and takes advantage of time needed to know vulnerability, fix it and distribute the fix. Attackers can use this time to spread malwares such as viruses, worms etc.
The only ways, to reduce chances of being attacked by Zero day exploits, are to follow security best practices, keep Security software and OS updated, and use caution while opening attachment, downloading files and installing software.
