All material things around us (and we ourselves) consist of molecules. Molecules consist of atoms, atoms consist of electrons and atomic nuclei. Behavior of all these small parts of materia is determined by the fundamental laws of quantum mechanics.
For molecular motion at "normal" conditions, even classical laws of Newtonian mechanics are valid. By knowing these laws, it is not difficult to predict what would happens with two atoms taken separately from the rest of the world. This can be done even with the help of pen and paper, as it was done before the second half of the 20th century. When the number of atoms in the studied system increases, calculations on a paper become no longer feasible. Here computers come to help. With modern computers, it is possible to compute the force acting on each atom in a system of many thousand atoms, and using the second law of Newton predict the motion of these atoms and thus behavior of the whole system.
In cases when precision of the classical mechanics is not satisfactory, laws of quantum mechanics can be used, though these laws require more extensive calculations. Oppositely, for larger molecular systems, some more simplified description than the atomistic level can be used. Since computer simulations use only the fundamental laws of quantum, classical and statistical mechanics, they are able to produce very accurate and detailed information about the studied molecular system, information which is often impossible or unacceptable expensive to extract from an experiment.
Also, computer simulations are indispensable in visualization of molecular motion and in interpretation of experimental data. Nowadays computer simulations are routinely used both in fundamental and applied research, such as drug design, development of new materials and technological processes.