Banding is an old ability once considered part of the main game, but is no longer supported. Fifth Edition was the final Core Set to employ it, and Weatherlight was the final Expert-level set to use it.
Banding works during the Combat phase, and it works differently depending on who's attacking. Essentially, it lets multiple creatures attack/block as though they were one creature. When you attack, you can add any number of banding creatures onto one attacking creature (meaning your band can only have 1 non-banding creature). If one or more of your band's creatures has an evasion ability, such as Flying, Fear, or Shadow, but at least one creature in the band lacks that ability, then the entire band can still be blocked as though the ability didn't exist; in other words, a creature that could block any member of the band can block the entire band. As usual per normal rules, you can always block 1 creature with any number of creatures, but as long as 1 of the creatures has banding, the entire group blocking the creature is a band. As long as one of the creatures has banding, you can assign combat damage to your blocking creatures as you wish. This can cancel out Trample, as lethal damage has to be assigned to all blocking creatures before it can be assigned to the player. However, this only applies to the creatures blocking one particular creature. If your opponent attacks with 2 creatures, you block each of them separately, and one of your blockers has banding, you cannot split the combat damage between them because your blockers are blocking separately. If your 2 blockers blocked 1 of the attacking creatures, you would be able to divvy up the damage as you choose.