I don't get it. Wat happen? Did Ren like,use her finger nail and made an X on miku's hand or some thing? Does it have anything to do with http://danbooru.donmai.us/data/9758e366f4701d819fce20756e18b90d.jpg ?
Some1 mind telling me about this?

She tripped when trying to swat the mosquito, and then got bit. And it's itchy.

The end.

And yes, Rin used her finger nail to make an x shape on top of the mosquito bite. This is supposed to make it feel better, but it's still itchy in the 4th panel.

I used to do that to my bug bites when I was a kid. It worked for an hour or two, then got itchy again.

people tell you not to poke at it, but you do it anyway.

i use to make imprints on my bug bites too,

Its an old myth that making an "x" on a mosquito bite with your fingernails makes it stop itching.

This is probably just making fun of people who beleived that.