What does it mean to try to use 'FindFirstChild' to index nil in Roblox Studio?

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When you use 'FindFirstChild' to index nil in Roblox Studio, it means that you are trying to access a child of a parent that does not exist. This error occurs when you try to search for a child object within a parent object that is not currently in the game. To fix this error, you need to make sure that the parent object exists before attempting to access its children.