What does this mean: “When a creature enters the spells area for the first time on a turn” ?

@JeremyECrawford "When a creature enters the spells area for the first time on a turn". What does this mean in relation to moonbeam?

— Regan Knight (@Day_Knights) March 25, 2016

Moonbeam is meant to get you when you enter the light (not when it passes over you) or start your turn in it. #DnD https://t.co/f5fyucucqn

— Jeremy Crawford (@JeremyECrawford) March 25, 2016

@KhellionDaxI’m with Critical on this one. What makes Moonbeam different from other spells where is says at the start of your turn? I'm not sure what you're asking. Moonbeam does, indeed, get you if you start your turn in the light.

— Jeremy Crawford (@JeremyECrawford) March 25, 2016