"Three lines" means girl?

Grace • cant wait to meet my little lady 🎀
I keep seeing people say if it's a girl you will see three lines and I'm so confused by that! I found out I'm having a girl today, but I don't see three lines or even understand what that means, can someone explain it to me? Im not doubting that mine is a girl cause theirs clearly no man parts there haha, but I don't know what lines people are talking about!

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