If you look at the atoms that make up everything or anything, they are moving around (more at higher temperatures) and feeling forces acting on them due to their neighbours (attraction and repulsion).
This means that the atoms themselves have got kinetic energy (K.E.) and potential energy (P.E.). This is known as the atoms internal energy.
You need to know that the internal energy, U, of a substance is made up of two components.
U = K.E. + P.E. = Ek + Ep