I think you are referring to the female human
reproductive system. Women are born with all of
their eggs already stored in bodily organs called
ovaries. After women reach sexual maturity,
they will begin to ovulat approximately
every 28 days.
Ovulation is when a mature egg that is ready
to be fertilized leaves the ovary and travels
through another organ called the fallopian tube to
the uterus (the womb). If the egg has been
fertilized by a man's sperm, then the egg will be
an embryo and will attach to the lining of the
uterus. Each month, the uterus produces a
nourishing lining in preparation for the arrival
of an embryo. At this point, the woman is
pregnant and the embryo will grow into a fetus and
eventually a baby.
Of course, women don't get pregnant every
single 28 days when they ovulate because usually
the egg is not fertilized. If the egg is not
fertilized, then it will not attach to the uterus
and the uterus will shed the unneeded lining. The
blood and tissue from this shedding exits from the
woman's vagina and this is referred to as her
period or menstruation cycle.
There's a great video here originally from
KidsHealth.org that explains the process that I've
described above with an animation.