You mean a lot to them (they cherish you).

It means they really really like you.

Long ago, the pupil of the eye was thought to be solid and was called the apple.

You should feel SO cherished, because being the apple of someone’s eye
is exactly that. I often tell my daughter that she is the apple of my eye and I
mean it with all the love in the world.

What does it mean? It means, you’re the shit- or at least to whoever says this about you. If you also meant to ask where the saying comes from, then my guess would be Shakespeare. “Flower of this purple dye, / Hit with Cupid’s archery, / Sink in apple of his eye” from A Midsummer Night’s dream. If the phrase was around before that, I don’t know of it. I think at the very least though he brought introduced it into the modern vernacular.

They adore you.

It means they adore you, they cherish you…you have center stage, so to speak. The saying did not originate with Shakespeare, it originated with God, who referred to Israel’s King David as the apple of His eye. He also called him a man after his own (God’s) heart.