An excellent article on the basics of pelvic floor dysfunction after birth is available on BabyCentre here:
https://www.babycentre.co.uk/a1014312/your-pelvic-floor-after-birth
PhysioPedia has a detailed description of pelvic floor anatomy if you're wondering what's going inside that's causing your symptoms outside:
This article from Tommy's is a quick read & a great primer for pelvic floor exercises that will help strengthen you down there:
https://www.tommys.org/pregnancy-information/im-pregnant/exercise-pregnancy/pelvic-floor-exercises