Zero-day exploits are hacking tools which exploit a previously unknown security vulnerability. These tools are often found to be used by law enforcement and government agencies in order to access devices used by criminals.
Israel seems to have a need for one as they have been sending emails and letters to several security firms in the US and Europe. Back in 2015, Israel contacted many US security firms by sending out a letter trying to pay for zero-day exploits, Motherboard published the letter sent to these firms here.
Recently it seems Israel reached out to security firms in Europe as well. A European source shared the email sent by the Israeli embassy in Berlin. This email was sent out in August of 2017, about two years after the letters sent out to the US security firms, and sounded the same asking for zero-day exploits.
It seems the Israeli government didn't constrain their email to only security firms as it was also sent out to people who that weren't exploit developers as well. The source of the email told Motherboard that they are "miles from an exploit developer".
The source also seemed to be reluctant on working with Israel, because they were a foreign intelligence.
They told Motherboard,
I'd be happy to help my own boys out but a foreign one not so much
It's interesting that government agencies will outsource their finds for an exploit. We have seen this happen before in the case of the FBI looking to get into an iPhone, as they bought the exploit from a foreign source.