A shim is conventionally a carpentry/metalwork/etc. term for a piece of wood, metal, rubber or similar material you stick between two things to get them in the

A shim is conventionally a carpentry/metalwork/etc. term for a piece of wood, metal, rubber or similar material you stick between two things to get them in the right position relative to one another, to fill some space, basically. This is necessary because the context of installation for many physical building situations are not reliably plumbed. It's also used as a term for things like thin washers used in lens manufacturing to deal with manufacturing tolerance differences when aligning elements in each individual lens. It is a term adopted here because the "shim" in this case is a small simple thing (driver) sitting between two other components to fix a problem. That's a simpler way of looking at a real life shim: a small simple thing stuck between two other things to solve a problem.