The mini PCI-E slot is most common in laptops made between 2005 and 2015.
The link shared by ko888 nicely showcases some of the cards that connect to the mini PCI-E slot, e.g RAID controller, video/audio capture card, Ethernet card, LAN card etc.
Though, to know exactly which connector your laptop has (mini PCI, mini PCI-E or M.2), look it up from your laptop manual.