A Firepit allows for the safe burning of wood, 200 at a time, with the help of flint and tinder to start the fire. Fish and vegetables can be grilled over the fire. Additionally, some items can be burned in a Firepit to produce ash or lime. It is possible to leave the byproducts of firepit usage(charcoal, lime, ash) inside the firepit and still be able to use it.
The residue of a Firepit is charcoal. Serious production of charcoal is more efficiently done at a Charcoal Hearth, though it requires more attention.
To build a Firepit, you must have learned the Controlled Burn tech at a UBody.
To build Self->Projects->Charcoal->Firepit.
To use a Firepit, you need the Firebuilding skill from a School of Leadership. You also need 200 Wood, 1 Tinder, and Flint to start the fire.
Firepits always produce Charcoal, though not as efficiently as a Charcoal Hearth. More ingredients can be added before lighting for additional results.
Once a fire has been lit, you have the option of stoking it if you have a Sharpened Stick or an Iron Poker. If you have both, it will let you choose which to use. Both seem to act identically other than that the poker does not seem to have a chance to burn up. Stoking a fire correctly keeps it burning longer, which increases its charcoal, ash, and lime yields. Stoking does not affect grilled food yields.
Consult the Firebuilding guide for more details and research into Firepit use.