A simple way to describe the various schools of philosophical thought is to examine how each school would view a simple object such as a chair. An idealist would view a chair as not being real. A chair can be dismantled or burned and would no longer exist. However, what is real is the concept of a chair that exists in the mind. If a thought or idea exists in the mind, it is real.
Realism is the second oldest school of philosophy.
Realists believe your mind can imagine things that don't exist and it is possible to arrive at incorrect conclusions through mental reasoning. Therefore, realists reject idealism and believe what is real is what is actually experienced. By studying actual objects and naturally occurring phenomena, we learn the truth. Realists favor the sciences. Realism didn't catch on until the 1600s.