This is not mentioned at A level, but is an important concept.
Energy is spread equally between different degrees of freedom.
A degree of freedom is a way of absorbing energy. There are three important types of degrees of freedom:
Translational: the ways the centre of mass of the molecule can move. There are often three translational degrees of freedom; in the x, y, and z directions.
Rotational: the ways the molecule can rotate. A linear molecule has two rotational degrees of freedom, and a nonlinear molecule has three.
Vibrational: all the other motion of the molecule. This is related to the molecule bending and stretching. Linear molecules have 3N-5 vibrational degrees of freedom and nonlinear molecules have 3N-6. N is the number of atoms in the molecule.
Each degree of freedom has:
Where kB is the Boltzmann constant.