In general, it translates instructions into something readable by whats accessing it. A popular translation layer on Lemmy is Proton. Its how the Steam Deck can play all those windows games.
Cuda is an Nvidia specific method for using a graphics card to do computation (not just graphics), like physics simulations.
Translation layers would let you use software designed for other graphics cards to work with Cuda, or to let Cuda software work on other graphics cards