It usually something relatively easy to make, especially for casual threads, and pertains to crafting your character for the roleplay. Its like an application of sorts, if it's accepted than said player can than post in the IC (In Character) part of the roleplay.
An example, you don't have to use this if you don't want to;
Name:
Age/Gender/Appearance: (Some people like to use photos, others like to actually describe their character's appearance in detail. Whether vivid or not, here's where we get a rough idea what your character looks like. Some people like to put age and gender in seperate parts of the sheet.)
Equipment/Weapons: (A general sense of the physical things your character carries. Sometimes they contain things of an emotional attachment, like pictures or a necklace. Weapons are also usually put under this section.)
Personality: (Simple enough, a brief description of how your character acts, their emotional state, things of that nature.)
Biography: (The length and detail of this is up to the GM (You, in this case) and involves recent events of your character's ongoings in the world.)