You can think of fixed costs as costs that you can't control the amount. Your rent is a fixed cost - if you're on vacation for half the month, that doesn't mean you only owe half your rent.
Variable costs are those that you can control. Once again, if you're on vacation half the month, you'll probably use less water, electricity, and gas.
Hope this helps - I know I hated doing breakeven points, they can be so confusing!
