A combo in Tibia can relate to either multiple participants of a battle firing their attacks at a target at the same time, which can also be multiple monsters attacking a player, or a creature casting multiple abilities at the same time.
History
The combos in Tibia found their origin on Antica, when player killers would use illegal macros to execute combos on unsuspecting targets.
Modern-era
The combos on modern era almost uniformly involve co-ordination with a voice-chat software, such as Ventrilo, Teamspeak, or Discord. This allows for not only accurate timing, but privacy for the participants as well.
Combos are the centerpiece of all modern wars. Battles are typical won by those who can execute combos most rapidly and efficiently, and can align themselves to perform combos more advantageously.
Stacking and Combos
Stacking is the act of placing many characters on a single square, using ramps, stairs, ladders, holes, or even doors. Stacking allows for a greater-than-average strength combo to be performed, which is a primary concern for both designers and players alike. It often provides an advantage to even the most unskilled of teams, and is rampantly abused on all of Tibia's PvP and PvP-E worlds.