Much of peoples' idea of chivalry is tainted by the fairytale version of history (knights in shining armour and so on). Chivalry is complex and involves the set of rules by which knights would not only do combat but also lead their everyday lives. All SCA fighters are required to behave in a chivalrous manner (see Rule 6 of Rules of the List). Everyone else should aspire to being chivalrous as well ;)
'Chivalry' is one of the ideals in which SCA aspires to taking the best of medieval life and recreate it. Essentially it is respect to others (and yourself), humbleness, graciousness and courtesy.