A capture card takes a video input, usually HDMI, and then gives out to you back in a form that can easily be manipulated by software like OBS or a video editor. There are two types: external and internal. Internal capture cards connect via PCIe to the computer; external capture cards connect via USB to the computer.
The input is what you want to record. The output is for displaying what the capture card sees. They're usually used to record things like consoles, not really the computer, though I think you can make one record the computer as well.