Just like in the real world, you need directions for how to get from A to B. In the world of computers, we call those directions “drivers.” MIDI drivers are bits of software code used to enable a MIDI interface to be recognized by the computer’s operating system so it can be utilized by other programs and have the appropriate data routed where it needs to go in an understandable format.
These days, most USB MIDI interfaces use the MIDI driver built right into your computer’s OS, but some interfaces that are either more feature-rich or older require you to install MIDI drivers specific for that device. Drivers are often updated by the manufacturer over time, and you should make a habit of regularly checking the support section of the manufacturer’s website for updated drivers.