I guess a lot of things, I guess. Seems silly to call something as simple as tickling complex.
But there's playfulness: giggling and poking and generally being silly with each other.
There's comfort and trust: I'm not going to just let anyone tickle me, and I wouldn't just tickle a stranger. Being able to open yourself up to that involves being comfortable with the situation and trusting your partner to respect your body and wishes, and you theirs.
And there is power: I'm not into the whole domination and torture end of the spectrum of tickling at all, but its still there. The ability to make someone's body react so much by the simplest touches is amazing. To be able to make someone laugh and wiggle with the softest, simplest strokes is kind of crazy.