Comey said "we cannot find a case that would support bringing criminal charges on these facts." What does this mean?

It means that there has never been a case in the past that has been taken to trial to test the law in a similar way. Or there is no "Precedent". It's a very technical excuse. The law in question is the Espionage act of 1917, and while it has been used successfully against low level people, it has never been used against a cabinet member. If the prosecution would fail, then the Espionage Act of 1917 might be thrown out, and that would free a lot of spies.

In other words, in US history, the Secretary of State has not been indicted for espionage, and Comey doesn't want to risk a loss. That's what's going on here.

It would be a lot more clear if people would name the law that has been violated, which is "the Espionage act of 1917".

The question is Why does Hillary act like a spy? Comey isn't sure what to do because she has the presumption of innocent, so he keep saying, Hillary was careless. Otherwise he'd have to say, she's a spy.