Basically it means a kit that has been produced, generally, in small numbers by an individual or small group of individuals who are not affiliated with what we would think of as a large company or manufacturer (Pegaso, Andrea etc.). The term "garage" comes from the fact that some, or most kits are made in someones "garage" or home workshop and not in a formal manufacturing facility (brick and mortar shop or location) as we might imagine it. HTH,
Jay H.