It’s the number of the signal sent, 9 is for SIGKILL. You can send various signals with kill, and depending on how application was made it may react on all signals with dying, or meaningfully process most of them. Afaik, SIGKILL can’t be processed by the app, and it always means just that: “die already”.
Checked in Wikipedia, that’s about right but there are more details I left out, mostly because didn’t know about them, too: POSIX signals