Quickie: explain php ->

Hi

not getting any joy on other topics but this is a real quickie so hopefully I can get some help...

Digging into the innards of buddypress I keep coming across "->" as some kind of operator that I can't find any documentation for anywhere.

Can someone explain what an expression like this means?

$bp->active_components[$bp->current_component]

Something to do with arrays, not clear what.

Thanks!