A row in your project budget. Budget lines are organised in a tree: high-level categories like Foundations or Roofing split into smaller items underneath.
Each line carries an estimate (what you think it'll cost), a budget (what you've committed), and links to the invoices and quotes that hit it. The budget view rolls them up so you see, at every level, where you are against plan.
Most budget lines come from Hemma's canonical template. You can add your own where the template doesn't cover what you're building.