What Is A Zero Day Exploit?

what is a zero day exploit, and why is it a big deal to put four of these exploits into one piece of malware?
im curious after reading this on the bbc
”n addition, it exploited several previously unknown and unpatched vulnerabilities in Windows, known as zero-day exploits.
“It is rare to see an attack using one zero-day exploit,” Mikko Hypponen, chief research officer at security firm F-Secure, told BBC News. “Stuxnet used not one, not two, but four.”
He said cybercriminals and “everyday hackers” valued zero-day exploits and would not “waste” them by bundling so many together.”