Someone who can't think calmly and always jumps into action without thinking much. They're rather unpredictable for both their allies and their enemies.
Their motto is: "Act before thinking." They generally don't care about details and get into the action instantly. It's quite common in anime and often used term for both males and females. Although it's clearly more associated with male characters and considered a manly action. As they rarely put much thought into anything they're generally considered idiots and suspects to traps or other hidden threats.
Being hot-blooded is usually the sign of strong willpower and in fightingshounen stories every important character tends to have at least one hot-blooded moment. Being hot-blooded comes with highly emotional speech(usually in the middle of battle...you know: "talking is free action ") and moments of determination. Those moments are often associated with surpassing one's natural limits or even GAR.
Nowadays classical hot-bloodedness became cliche and the classical hot-blooded character degraded into a comical one. Although if anything Tengen Toppa Gurren Lagann shows that this kind of concept still has a bright future ahead. The concept constantly evolves and creates many intermediate and sub-categories.