Combos are an added bonus for melee weapons. By using both an action and bonus action and by succeeding some sort of ability check or saving throw they weapon can deal extra damage or have a special effect. Combos can only be used 2 times per long rest. The type of combo depends on the weapon:

Heavy or Two handed 

The player swings the weapon in circles, gaining momentum with each revolution. The player can move up 10ft while spinning. This attack targets all creatures in a 5ft radius of the player. Each creature in the radius makes a competing dexterity save with advantage to the player's attack roll. On a success, the creature takes half damage.

Spear-like

The player makes a series of attacks using each part of the weapon. The player must make 2 attacks with advantage, these attacks deal half damage and any damage modifiers are only added once. The 3rd attack has disadvantage and on a hit does full damage and makes the creature fall prone.

Light

The player makes one very strong attack with the weapon. This attack has disadvantage, but deals double damage on a hit and stuns the opponent for 1 round.

Any questionable weapon categories like versatile weapons can do any combo that the DM decides fair.