Mindlessness is doing things without thinking them through in a careful way. Your mindlessness might lead you to say something insulting about a classmate, not realizing she's standing behind you.
You can use this noun to describe an empty-headed lack of meaning: "The mindlessness of that show made me feel ten percent more stupid when it was over." But it's also useful for a more dangerous kind of rash, thoughtless action: "Her experience as a soldier exposed the mindlessness of most wars." These days, many people aspire to mindfulness, a state of close, active attention. You can think of mindlessness as the exact opposite.