What is Temperature? Well lets start with a Definition :-

Temperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter, and a material with a lower temperature is said to be cooler. Temperature is a measure of the average kinetic energy of the particles in a sample of matter.

Let me explain Our Planet is made up from matter which is everything around you. Matter is made up from different Elements in various states Solids, Liquids, Gases, Plasmas, and a new one called Bose-Einstein condensates, but we won't go there for now. All of which can be heated or cooled.

These Elements are made up of Atoms. For example the two main Elements in air are 78% Nitrogen, and 21% Oxygen.

The atoms in the air are constantly vibrating and moving around, and as they receive energy from a source such as the Suns radiation they will move quicker. The quicker they move the more they bump into each other causing them to release Thermal Energy (Heat).

When the air cools this means less energy is being absorbed so the atoms slow down, and therefore create less Thermal energy so Cools. That is why it cools at Night (Less Energy from the Sun), and it warms up during the day (More Energy from the sun. Elements will expand or contract as they are heated or cooled, which is why air temperature has a direct effect on Barometric Pressure.