cutout on side of unibody macbook pro

ExpressCards allowed one to connect a variety of devices to a computer including modems, SD cards, FireWire connectors, USB and Ethernet ports, external HDDs and SSDs, TV tuners, sound cards, graphics cards and other peripherals devices.

That's an ExpressCard/34 slot. It is shown in the user manual.

I've never seen an actual ExpressCard in real life.

smidgeondutchrabbit wrote:

this is model mb604 to md331. (17 inch unibody model) also what is an expresscard slot used for

It's just an I/O port, like USB or Thunderbolt, for connecting external devices.

The cards tended to be smarter than just a cable connection, more like an adapter and possibly cable in one. So more like an Ethernet or Wi-Fi or card reader Adapter plugged in there, not just an Ethernet cable.

Another way to think about it was the dongle part went into the slot, and just the cable (if any) connected to that little card.

For more information than you probably wanted, see:

https://en.wikipedia.org/wiki/ExpressCard