When you tickle someone, you touch them in a way that makes them laugh and squirm. It's impossible to tickle yourself—you can only tickle others.
You might tickle a baby to make him squeal and giggle, and the sensation he feels is also called a tickle. An itching sensation is also sometimes referred to as a tickle, like a tickle in your throat when you're getting sick. Colloquially, you can tickle someone (or tickle them pink) by delighting them: "It tickles me when you surprise me with cupcakes!" Some experts suspect that this word comes from the Scots kittle, "to tickle."