What causes a resting membrane potential?

Membrane potential is the difference of charges between 2 sides of a membrane. Thus the resting membrane potential is the "normal" amount of ions on both sides of the cell membrane. Inside most cells you have many proteins (negative charge) and Cl-. Outside the cell you have a ton of Na+ / K+ waiting to rush in. Most importantly are Na+ and K+. The sodium potassium pump is largely responsible for depolarzation which is caused by the positive charge influx.

Most cells at rest have a negative polarization. So when it becomes positive they are "depolarized."

They become repolarized when the Na / K pump removes them from the cytosol of the cell back to the extracellular fluid.

I have been out of classes for a while but ill see if i can explain it a bit..

The difference in membranes has to do with the chemistry of the fluid on either side.

Resting membrane potential allows the fluids to openly mix through the membrane so if you have pure water on one side and salt water on the other, the sodium and chloride will cross the membrane to turn the pure water into salt water so now they are evenly mixed and the same solution on either side..

Action potential implies there are mechanisms (i forget the actual name ) within the membrane (like a living cell) that actively transport chemicals in either solution across the membrane. ie: the mechanism in the membrane wants as much sodium inside the cell as it possibly can so it actively transports it through the membrane..

I hope this helps.. at least to understand what the book is saying, im sure it is in more detail.