Fixed costs
Expenses that stay the same each period regardless of how much you use, buy or produce, such as rent, insurance premiums and loan payments.
Fixed costs are the part of a budget that this month's behaviour cannot touch. Rent or a mortgage payment, insurance, a car loan, a flat-rate subscription: the amount is set by a contract, so it arrives whether or not anything changes. A business uses the same idea for rent, salaried pay and equipment leases, costs that do not move with the number of units sold.
They matter because they set the floor. Their total is the minimum a month has to bring in before anything else is possible, which is why an emergency fund is usually sized against essential costs rather than against total spending. In a business the same total is what has to be covered before the first unit of profit, which is where the break-even calculation starts.
Fixed does not mean permanent, and that is where budgets stall. A fixed cost is fixed within the period, not forever: rent until the lease ends, insurance until renewal, a phone plan until you switch. Trimming groceries week after week is slow, repetitive work. Renegotiating one fixed cost changes every month that follows without another decision being made.