Bo5 is short for "best of five". It denotes a series in which two teams play up to five maps, and the winner is the team that is first to reach three wins. If one side wins the first three maps in a row, the series ends 3:0 and the remaining maps are not played. The maximum length of a series is five maps, at a score of 3:2.
How the series works
The score in a series is counted in maps won. The possible results are 3:0, 3:1 and 3:2. The deciding fifth map is needed only when the score is level after four maps, at 2:2. Such series are usually considered the most tense by viewers: the cost of every mistake on the last map is at its maximum.
Most often, maps in a Bo5 are chosen through a pre-agreed procedure. In games with maps, such as CS2, teams take turns banning and picking maps, and the last remaining map goes to the fifth game. The exact order depends on the tournament rules, so it is always worth checking the competition regulations.
Why this format exists
The main advantage of Bo5 is accuracy. In a single map (Bo1), chance decides a lot: a bad start, an unexpected play, one failed economy gamble. In a first-to-three series, such episodes are smoothed out: to win, a team has to show consistent results across several maps rather than one lucky day.
In addition, Bo5 tests the depth of a team's play. A team has to play on different maps, and a weak spot, for example a disliked map, sooner or later comes to the surface. That is why a wide map pool and the ability to adapt to the opponent matter more in a long series than in a short match.
The downside is time. A five-map series can last several hours, which is a strain on the players and a scheduling challenge for the broadcast. That is why Bo5 is used selectively: in finals, semifinals and decisive playoff matches. In group stages and early rounds, where there are many matches, Bo1 and Bo3 are more common.
Where the term comes from
The notation "Bo" followed by a number comes from English and has become established in esports as a universal one: Bo1, Bo3 and Bo5 are read "bo-one", "bo-three" and "bo-five". In the Russian-speaking community, people say "bo-five", "the five" and simply "a series to three wins". The format is not tied to any one game: it is used in shooters, in MOBA games and in other disciplines where a match consists of several separate games.
A practical example
Imagine two rosters, one of which is stronger on its "own" maps but noticeably weaker on the rest. In Bo1 it can win if it lands on a comfortable map. In Bo3 the opponent already has the chance to ban its favorite map and impose its own. In Bo5 this difference shows up most strongly: to win, a team has to take three maps, which means at least one of them will have to be won on less comfortable ground. That is exactly why the winner of a Bo5 is more often the team with no obvious weaknesses.