Small molecules like water, salt, and sugar can pass through this membrane, but larger molecules like proteins cannot.
Through a process of osmosis by diffusion, the cell moves water, salt and sugar in and out of the cells trying to get things back into balance with the surrounding liquid.
Here's a crude illustration that I've drawn to show what I've described.
I hope this gives you a better understanding of how brining works, and why it's a wonderful way to let nature help you cook a much better bird.