The Bandolier is a new item in Risk of Rain 2.
Upon pickup, it gives enemies a chance to drop an Ammo Box on kill, which refreshes all skills currently on cooldown as if they were never used. For example, if the player is using Huntress and has just used a skill such as Ballista, instead of waiting for the 17 second cooldown they can just collect an ammo box and instantly regain the ability to use the attack.
The drop chance is affected by Luck.
In Steam Build 3678136, the tooltip droprate is false, as it is 20.4% and the second bandolier you pick up is only a 10% increase. After the first two, the increase in droprate quickly diminishes. The actual droprate is given by 1-1/(bandoliers+1)^0.33. The actual droprates are as follows: