“Staff” has been a regular title for SV software companies for quite a while now.
It is basically the next level past “senior”. Whereas a senior will normally report to a line manager, staff engineers will often report to a middle manager and have managers as their peers.
A normal range for a senior is 5-10 years of experience (but still many people will have more experience than that but not be “senior” depending on company), whereas staff is 10+ years. Obviously these are not wholly sufficient conditions but they’re common buckets, especially for external hiring.
There is some title inflation going on where staff engineers are typically acting more as tech/team leads as opposed to senior engineers. IMO it’s a good title because “principal” has connotations of doing really big-picture stuff that may involve some resting on laurels. Also the bar for “principal” varies widely across companies - at Microsoft it’s the same as “Staff” at other places, at some places it’s like the top <1% of IC engineers, others are probably even more lax than MS.