Idealism is the opposite to Nominalism. It considers ideas or concepts - e.g. the idea of "blue" or "triangle" to exist as valid entities in themselves, quite distinct from phenomena and words (Nominalism in contrast denies that ideas or concepts exist apart from the physical phenomena of the senses). In short, it adds a "spiritual' or "metaphysical" dimension to the material.
These unchanging archetypes or idaes were often posited as thoughts in the mind of God (e.g. pre-Christian and Christian Platonism).
Idealism as defined here, or Objective Idealism, should be distinguished from Subjective Idealism, in which things only exist in the mind of the person experiencing them.