To elaborate... a combo is an inescapable string of hits - Two or more consecutive hits where the opponent does not leave hitstun. True combos are inescapable regardless of DI, while some "combos" can be avoided by getting out of the range of the second attack, although they will still cause the combo meter in training mode to go up, because they were still in hitstun. However, seeing as hitstun is so low in brawl, few combos actually exist. Most "combos" that you see in brawl are actually just strings. A string is defined as an avoidable series of consecutive hits. More often than not, a character will have left hitstun before the attack that hit them has even ended, allowing players to easily airdodge out of any consecutive hits, or even attack a player before they have a chance to hit them again.