The matter can mean both an abstract body (if it is about definition from a physics course), and very concrete body, human. Let's go from the general to the particular...
From a school course of physics we know that body temperature characterizes a condition of thermal balance and is an indicator of kinetic energy of molecules of this body. The quicker they move, the body temperature is higher. With temperature change also properties of a body can change (we will remember water: frozen, it represents ice, and heated — steam).
But what it can do means in relation to a human body? What is characterized by temperature of a human body? Most often — state of his health.
We got used that temperature increases during a disease. Microbes, getting to an organism, emit poisons, because of it in an organism the substances operating on the temperature center of a brain begin to be produced. In this case he perceives normal body temperature as lowered and increases it. For this purpose the body begins to protect heat due to narrowing of vessels, decrease in sweating — we turn pale and we chill. As soon as temperature reaches a certain point, the organism holds it, having ceased to save heat therefore vessels extend, the pallor and a fever disappear, skin becomes hotter and we feel heat. As soon as microbes are terminated, the organism seeks to return to usual temperature: sweat is plentifully emitted, the body gives a lot of heat, will not return in a normal temperature mark yet.
There is one more point of view on temperature increase of a human body during a disease: it is considered that so the organism fights against microbes, actively developing antibodies, not allowing harmful microbes to breed. Therefore to prevent it to struggle with a disease too is not necessary: febrifugal medicine begins to be taken if temperature rises higher than 38 degrees at adults, 37.5 degrees at children. If the health worsens also at lower temperature, it is not necessary to postpone drug intake too. Except a disease, body temperature rises during physical activities: as is well-known, a great way to be warmed on the street — active movements, games or warm-up. Also temperature can "jump up" because of nervousness, because of fear, and still — at mental work. From a stress temperature can both increase, and to go down. Fall of temperature can also speak about decrease in immunity, a lack of vitamins or physical exhaustion, about chronic fatigue. And still fall of temperature is one of signs... pregnancies. If body temperature is constantly lowered (about 35 degrees), it can speak about a disease too. Maybe so that this temperature for the person is normal, "working": he feels perfectly with such temperature many years. But before taking it for norm option, it is better to undergo medical examination nevertheless.