In its plasma membrance, axons have something called a sodium-potassium pump. This constantly moves sodium ions out of the axon and potassium ions into the axon againsts the concentration gradient (Active transport ----> large amounts of ATP required. AS synoptic content). The sodium-potassium pump removes 3 Sodium ions (Na+) from the cell for every two potassium ions (K+) pumped into it. The plasma membrane of axons is slightly more permeable for potassium ions than sodium ions so some k+ ions diffuse back out.
As a result of this, there are more positive ions outside the membrane than inside the axon setting up an electrochemical gradient. This means the outside of the membrane is more positive than inside it (as well as there being a concentration gradient).The difference in charge on the two sides is called the resting potential. It is mostly at about -70mV (millivolts) on the inside compared to the outside.