A Default is when a team splits up into watch different angles and "Flanks" across the map. This is used in many popular FPS games, seen often in Valorant and CS:GO.
In maps like 'Split' on Valorant, you would likely want a 2-2-1 formation, 2 A, 2 mid, 1 B. This allows for many primary angles and flank routes on said map to be covered by the team.
The Act of defaulting especially helps with making sure your team doesn't send all 5 players into a site, then get flanked from 3 angles, then lose the fight.
Hey, we're on (this map), we should default to (this site, this site, and mid).