When you smell something, that thing is actually inside you. You can say you smell a flower, but what you’re really smelling is some volatile molecules from the flower, and when you smell them they’re already inside your nose.
Likewise, when you hear, the sound waves must have already entered your ear.
Even when you see, what you really perceive is light hitting your retina, which is inside your eye.
But when you touch something, that thing is still outside of you. Technically, can also be said of taste if you lick something, but then there’s a strong likelyhood of some molecules of the thing entering your mouth as a result.
The Light is what is being perceived in that example. The point of mentioning that the retina is inside the eye is that it means light must be inside the eye too when it hits it.