Bo1 is the shortest of the tournament match formats. The "Bo" prefix is short for "best of." The number after it shows the maximum number of maps a pair of teams can play. In the case of Bo1, there is exactly one map: whoever wins it wins the whole match. There is simply no room for a second or third map.
How it is read and written
The term appears in several variants: "bo-one," "bo1," "Bo1," and sometimes with a space, "Bo 1." In conversation and on broadcasts, people most often say "bo-one" or "bo-van." A match might be described as "playing Bo1," "it's a Bo1," or "a Bo1 bracket."
Why organizers choose Bo1
The main advantage of the format is speed. A match takes a single map, so many matches can be held in one day. This is convenient where there are many teams and little time: in early stages, large group stages, open qualifiers, and qualification rounds. The broadcast schedule is also easier to plan when the length of a match is almost predictable.
The price of speed: randomness
Bo1 has a downside. On a single map, the result depends more heavily on luck, on a team's form on the day, on an unexpected tactical choice, or on a map that a team has prepared specially. A lower-rated opponent can "steal" a win from a stronger one: it needs only one good map, rather than two or three in a row, as in longer series. For this reason, the outcome of a Bo1 reflects the real difference in team class less accurately than the outcome of a Bo3 or Bo5.
For the same reason, preparation is especially important in Bo1. Teams often work out in advance exactly the map they will get in the match and rely on prepared material: rehearsed utility setups, plays, and surprise strategies. One well-prepared "surprise" can decide everything, because the opponent has no second map on which to adapt and fight back.
A typical example
Imagine a group stage where eight teams play each other once. If every match were a Bo3, the schedule would stretch over many hours. In Bo1, all the matches fit into a reasonable amount of time. On the other hand, it can happen that a team regarded as the favorite loses on a single map and is eliminated or drops a point, while in a three-map series the outcome that was entirely expected might have been different.
How the map is chosen in Bo1
In longer series there is usually a map selection procedure in which teams take turns banning and picking battlegrounds. In Bo1 the map is also determined by the rules of the specific tournament: for example, the opponents take turns removing maps from the pool until one is left. The exact order depends on the regulations and may differ between tournaments.
Bo3 and Bo5 nearby
Bo3 is a series played to two wins, and Bo5 to three wins. The longer the series, the more accurately it shows the strength of the teams, but the more time it takes. Bo1 sits at the opposite end of the scale: maximum speed and maximum randomness. That is exactly why it is rarely seen in important matches, especially finals, and often seen at the start of a tournament.
For a viewer, the Bo1 label in a bracket or schedule is a signal: an upset is possible here, and any favorite risks being knocked out after a single bad map.