Few keywords have confused more magic players new and old than banding. When attacking, any number of creatures with banding, plus up to one creature without it, can form a band, and if any member of the band is blocked, the whole band counts as blocked. The real payoff comes in combat damage: whenever a creature with banding is blocking or being blocked, its controller decides how the opposing creatures' damage is divided, not the opponent.
Benalish Hero and Mesa Pegasus bring the keyword cheaply, while Helm of Chatzuk grants banding to any creature at instant speed so a bigger threat can join in. A few cards use a narrower mechanic called "bands with other," which only lets creatures of a specific kind band together. Wizards retired the keyword long ago, so today banding is mostly a nostalgic pick for brewers.