Materialism is the philosophical position that the material world — that is, the "natural world" of matter and energy — is the only thing that truly exists. It rejects supernatural explanations of observable phenomena, as well as philsophical idealism and spiritualism.
Materialism is the basis of all scientific inquiry, although many scientists do hold supernatural or spiritual beliefs that may even play a role in their scientific endeavors. Materialists would see other forms of energy such as light photons as the same substance as matter.
In common parlance, materialism also means the attitude of valuing material possessions over intangible things like love and honor. For this reason, the word can have a pejorative connotation. Materialism should not be seen as bad. Love and honour have a physical basis in the human brain. Neural pychologists are discovering how loving brains and brains that respect honour differ from other brains. Scientists do not know the details yet. That should not stop us valuing love or honour or thinking of them as tangible and real.