Biodata is just what it sounds like - biographical details for a marriage! This is often used for arranged marriages so that a family can decide what they want for their son or daughter.
This biodata form consists of the following details:
Current city and city born in
Native language and spoken languages
Physical description, including height, complexion, etc.
Details about one's family (how many siblings, parental information)
Contact details such as phone number, address, and e-mail
Nowadays, it can also include details such as what kind of partner preferences that person has.
Once a family has the details of the prospective spouses, they can compile the information to find out which candidate is most suitable for marriage.
Still confused? I read an awesome article called "the real deal about arranged marriages" that can help clear this up!