Question

What does the Plasma membrane do

Answer 1

Separates the inside of the cell from the outside of the cell.

Answer 2

Aside from what blues said, the plasma membrane regulates which come in and out of the cell. It picks out what substances enter and leave the cell.

Answer 3

Actually, the plasma membrane doesn't actively select or pick out anything. Both selection of transported molecules and transport of those molecules across the membrane are mediated by proteins. Some proteins, like porins, enable diffusion down concentration (and electrochemical) gradients; others actively transport molecules against their concentration gradients and are at least partially responsible for formation of those gradients.

In essence, that is what the plasma membrane is: a barrier across which gradients can be formed.