What is the <number>! in byobu's status bar mean?

From man pages:

updates_available - the number of updates available on the system; displayed in the lower bar toward the right in white text on a red background with a trailing '!' sign; if any updates are marked 'security updates', then there will be a total of two trailing exclamation points, '!!'

This shows the number of packages that need to be updated on the system.