This is terminology used by the JET Programme to identify native speakers of a language employed to assist in teaching that language in schools in Japan.
ALTs can either be assigned to a single "base" school, and teach virtually all of their classes there, or be assigned to an office, and visit several schools on a rotating basis. ALTs at senior high schools (grades 10-12) tend to have a base school, while those teaching junior high (grades 7-9) and elementary (grades 1-6) tend to visit several schools.