One parser watches each chain. Every parser records the block it has read up to, at least once a minute while it is healthy, so the age of that record is what says whether the events below are still arriving.
Online means the parser reported within the last 5 minutes, degraded that it has been quiet for longer than that, and offline that nothing has been heard for over an hour. A chain marked as not reporting has no parser configured on this deployment. The block number is how far that parser has read, which lags the chain's own head by a couple of confirmations, and by more while a parser is catching up after a restart.