In Heroes' Ascent, the holding team is the team who is in possession of the Hall of Heroes. In a more general sense, holding usually means a team who holds the Hall of Heroes continuously rather than capturing it and losing it the next round.
The term is rooted in the idea that the Hall of Heroes must be captured by your team's Ghostly Hero. The holding team secured favor of the gods for their territory. If a challenging team defeats the holding team they have "captured" the Hall of Heroes and therefore secured favor for their territory.
During matches in the Hall, the holding team is the blue team. The holding team starts with a slight advantage in that their resurrection shrine is closer to the central altar allowing them to move to the center quicker after death. Additionally, in King of the Hill matches, the holding team's Ghostly Hero begins the match in possession of the central altar. In Relic Run matches, the holding team begins with a slight advantage because the gate out of their base is closer to the central altar. They can carry their relic to the gate and capture the first relic. (This advantage is only good for the first relic because the hold team's relic still spawns equidistant from the central altar compared to their opposing teams.) This advantage can also be seen as a disadvantage because,during Capture Points the blue team's base is easier to get to than the opposing teams making it easier for enemies to split off towards their base. Additionally, because their base is between the rival red and yellow teams, it is easier for the other teams to gang up on the holding team.
Holding can also be where one team seizes and defends an advantageous part of the map or arena as part of its strategy to win the match.