Can you briefly explain the concept of dam tohar adn why a women is not pure after birth?

The Torah (Vayikra 12) teaches that a mother after a birth of a boy will be Tameh – ritually impure, for 7 days and 14 days for a girl, after that period all bleeding will not render the woman Tameh, 33 days for a boy and 66 for a girl – that period is called Dam Tohar – ritually pure bleed. If during delivery she was a Zavah – certain type of feminine ritual impurity, the above does not apply and she will be Tameh until she counts the seven clean days, just like after her menstrual period. Nowadays we hold all women as if gave birth within the status of Zavah and the laws of Dam Tohar in a practical sense, are irrelevant today. (Rambam Isurei Biah 7, Shulchan Aruch YD 194, 1)