What does it mean for a boy to tickle a girl's left ankle?

If he catches it lightly, he likes you. If he grabs hard and hurts you, or doesn't make you feel a little afraid of being hurt, it's purely a joke. From the light scratch you said, on the one hand, it is also the expression on his face after grabbing your ankle. If he feels happy, excited or a little ashamed, then he likes you. If he looks like a hooligan, he is playing with you!