My formula is showing a #REF! error. What does this mean and how can I fix it?

A #REF! error indicates an invalid cell reference. This typically happens when cells or ranges referred to by a formula have been deleted, cut, or pasted over, causing the reference to become broken. To fix it, you need to either undo the action that caused the reference to become invalid or manually re-enter the correct cell references into the formula. Always be cautious when deleting or moving cells that might be part of existing formulas.