It means pseudo element selector. It means the element to the right doesn’t exist in the normal DOM, but can be selected.
A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.
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It was originally only a single colon, but was changed to differentiate it from pseudo classes (like :hover, :first-child, :not etc). It’s best to use : for before and after pseudo elements since the single colon has better browser support, namely in earlier IE versions.