In short: Fixed costs stay the same however much you sell (rent, salaries, subscriptions), while variable costs rise and fall with the volume of sales (raw materials, packaging, commissions).
Simply put: In a small business, variable costs are usually also direct, and fixed costs indirect. The split matters because the fixed costs must be "earned back" every month before there is any profit. The same applies at home: rent and loan repayments are fixed; food and fuel are more variable.
Example: An online shop pays €900 a month for storage and software (fixed) and €4 for packaging and delivery per order (variable). At 100 orders costs are 900 + 400 = €1,300; at 300 orders, 900 + 1,200 = €2,100.
Watch out: "fixed" does not mean "forever". Rent is a lever too — lower rent lowers the break-even point.
Source: Module 18, Chapters 1 and 4 · OpenStax, "Principles of Managerial Accounting", 3.2 — https://openstax.org/books/principles-managerial-accounting/pages/3-2-calculate-a-break-even-point-in-units-and-dollars