A 'map handicap' market means you're placing a bet on one team having an advantage (or a disadvantage) in the number of maps they are required to win.
If there's a best of three match between two teams, you can bet on Team A with a "Map handicap" of +1.5. Team A will already have a 1.5 map advantage, so they will only need to win one map to win the bet (they can lose 1-2, or win 2-0, or 2-1).
Alternatively, you can bet on Team B which has a -1.5 "Map handicap". This means that Team B starts with a -1.5 map disadvantage. So in a best of three scenario, Team B must win 2-0 for you to win the bet.