If I understand your question correctly, you are referring to the text portion of the profile. The minimum required is:
== Biography ==
[put information about the person's life here]
== Sources ==
[sources to support the information contained in the biography are required. It is desirable, according to WikiTree's style guide to employ in-line citations]
Basically, you should include any facts you uncover about someone's life along with where you got your information. Your sources should be such that anyone else could come along and find them to see for themselves. In the case of an online source, a link should be included. If it is something like a birth certificate in your possession, you can either upload an image if you like, or just state where you got so that someone else could get their own copy if they need to.
The biography should concentrate on genealogical information, but can include other significant information about a person. They way I think of it is regarding a famous person; more about their family, less about why they are famous.
Here is the link from the HELP pages: