A 'first map - round handicap' market allows you to place a bet on a team, giving that team an advantage (or a disadvantage) in the number of rounds they are required to win in the first map of a match.
This market applies to CS:GO and any other game that has round-based scoring.
If there's a best of three match between two teams, you can bet on Team A with a 'round handicap' of +3.5 for the first match.
Team A will already have a 3.5 map advantage, so they will only need to win at least 13 rounds in the first match (in a first to 16 scenario, or overtime) to win the bet. Team A can lose the first map 13-16, or closer, or win the first match, for you to win the bet. In other terms, Team A, can't lost by 4 or more rounds for you to win the bet.
Alternatively, you can bet on Team B which has a -3.5 'round handicap'. This means that Team B starts with a -3.5 round disadvantage in the first map. Team B must win by at least 16-12, or by a larger margin, for you to win the bet. In other terms, team B, will need to win by 4 or more rounds to win the bet.