It's called "bassamat al-Farah," the Smile of Joy and symbolizes the joy of martyrdom.
A lone woman in Iraq, wearing a traditional hijab without a niqāb (veil) walks into a crowd and stands in a long line of people waiting for a bus. She has leprosy, is very depressed and is about to commit a suicide attack motivated by the ideology of Islamist martyrdom (shahid). She believes her sacrifice will be rewarded in the afterlife. Under her clothing she is wearing a vest, full of explosives.
She knows no one in the line, but wants to kill as many innocent people as possible, using it as a tool for psychological warfare affecting the general population by causing death and serious injuries to civilians and non-combatants alike for the purpose to intimidate.
She looks at the man standing to the right. For her, she is glad he is an American soldier, on leave. She smiles at him, giving him the "Smile of Joy," he smiles back. She turns and looks to woman standing on her left. She has two young children who are staring at her. She gives them the "Smile of Joy." They smile back.
She catches the mother's eye and smiles at her too, giving her the "Smile of Joy" then casually presses the detonator.
Her mission has been completed.