The add-in-cards use the PCIe bus to add extra functionality to your system.
The most common form of add-in-cards are for desktop PCs, workstations and servers, fitting directly into a PCIe slot on the motherboard. These take the form of NICs, adding more or faster network ports than the motherboard has, or add extra I/O ports such as USB, Thunderbolt or Firewire.
The second most common form of add-in-cards are for laptops, fitting directly into a M.2 slot on the motherboard. These take the form of NICs, adding WiFi capabilities.