As usual, there are plenty of different Vitamin K that are divided into two groups, K1 and K2.
We get half of our need from our food (we absorb about 10-70 % of it) and the other half is produced by the bacterias in our intestines.
The vitamin is needed for the bloods coagulation and thats why it's also called vitamin K - after the danish word koagulation (it was a danish person who discovered this).
Newborns does not have a funcional intestinal flora so they can not produce vitamin K. Therefore the babies usually get a single dose of vitamin K to prevent bleeding conditions.
You can get Vitamin K deficiency when you take antibiotic as that destroys the bacteria flora.
You can find vitamin K mostly in dark green vegetables and vegetable oils.