An affirmation is something you say to yourself. Everybody uses them on purpose or accidentally. You get up in the morning, leap out of bed and proclaim, "I feel excellent". That's a positive affirmation. You drag yourself out of bed at first light and whine "I feel crappy". That's a negative affirmation. Both statements help to sustain the emotional state you're in.

Our self-talk, the things we state to ourself, is really crucial as it directly affects our conscious and subconscious brain. We hear everything we say to ourself. Get all the info you need here.