Why were women unclean for 7 days after giving birth?

So i read a passage saying "she who gives birth to a boy is unclean for 7 days" what is that about?

In the OT ceremonial law, discharges caused both men and women to be unclean. It wasn't necessarily the birth of a child that caused uncleanness, but the bodily fluids involved. Ultimately, Galatians (3) says the Law was a tutor to point men to Christ. The ceremonial laws were supposed to make men conscious of the fact they were sinners who were falling short of God's glory. Then they see their need for a once-for-all atonement to pay for their sins. Enter Jesus Christ.