We measure our temperature using a thermometer, which is a glass tube that has a bulb filled with mercury at one end. When you place a thermometer in your mouth or armpit, the mercury in the bulb gets warmed by your body heat, and it expands.
As it expands, it moves up the glass tube that has markings showing your temperature. If your temperature is above the normal level, it means you have a fever. The fever can be caused by some kind of infection- in fact; a fever is the body's way of fighting the infection.
You should see a doctor at once, so that he can prescribe medicines which will help your body fight the infection even more efficiently.