Wombo Combo is a term used, primarily, to mean the complete and utter destruction of another person or team, especially if it is done quickly. It is originally from a video of a Super Smash Bros Melee match (below), but has since grown in popularity and is used for many different games and situations to the same effect. There are a large number of Youtube videos that use the same audio track and overlay other situations, gaming or not, as a parody of the original video.
Usually when used in games these days, it is done so in celebration or as a taunt or bragging to the other team. Most of the time, it is just meant as trash talk and is not referring to any action in particular as a Wombo Combo.
Potentially related, it is also a Pizza Combination at Round Table Pizza with "Primo pepperoni, Italian sausage, linguica, bacon, mushrooms, tomatoes, artichoke hearts, green onions on zesty red sauce."