distal stimuli are objects and events out in the world about you.
proximal stimuli are the patterns of stimuli from these objects and events that actually reach your senses (eyes, ears, etc.)
Most of the time, perception reflects the properties of the distal objects and events very accurately, much more accurately than you might expect from the apparently limited, varying, unstable pattern of proximal stimulation the brain/mind gets. The problem of perception is to understand how the mind/brain extracts accurate stable perceptions of objects and events from such apparently limited, inadequate information.