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
In languages like C, your application code can register what is called a signal handler. These functions get called when the process receives a signal. You could do something like reload a config file for example, without the user needing to stop and restart the process.
Many here will be seeing this for the first time. For others it will be a fun reminder of something they haven't seen for a long time. Still others will find it to be the same old tired shit they're sick of.
When you see the same old tired shit on Lemmy (or any social media), just move on. There will be plenty that is new to you.