Someone who's heartless is inconsiderate and insensitive to other people's feelings. It would be heartless to smash a little kid's carefully carved Jack o' lantern.
A heartless person might respond glibly to a friend's sad story about his sick grandmother, or push a hungry cat out the door on a rainy night. If you're heartless, you're cold and uncaring. Another, older meaning of this adjective is "lacking courage," from the Old English heortleas, "dispirited or dejected." The modern meaning was coined by poet Percy Bysshe Shelley in 1816.