They are different moments of consciousness which can be directly experienced with they appear. The Buddha spoke about realities which can be directly experienced and there are different from abstract ideas and imaginations. Seeing is the experience of what presents itself through the eyes, of what is visible. Seeing is different from thinking about what we see, different from attachment. Seeing just sees.
Hearing is the experience of what presents itself through the ears, of sound.
Hearing is different from thinking about what we hear such as someone's voice or the barking of a dog. Tasting experiences flavour. Flavour can be directly experienced when it presents itself. Flavour itself does not experience anything, it is different from tasting. The reality which knows or experiences something, nama, the reality which does not know anything, rupa.