Usually that means adding city to KW. So instead of just just optimizing for Plumber you optimize for Atlanta Plumber.
Depending on the context, or what you are optimizing for, the local modifier could be city or city state (Plumber Atlanta Ga) or county (Orange County Plumber). Sometimes even a regional name - like if someone wanted to rank for the Tri-city Plumber.
Often people refer to it as GEO modifier - same thing as above.