Sell one unit and you take in its price but pay its variable cost. The gap is that unit's contribution margin — what it puts toward the bills.
Contribution first has to pay off the fixed cost. The volume where it's finally paid off — where the revenue and cost lines cross — is the break-even point.
Trade fixed cost for variable cost and the cost line pivots around your target: the same profit there, but a different break-even and a different risk.
That sensitivity is operating leverage. A high-fixed-cost firm earns richly above break-even — but its margin of safety, how far sales can fall before losses, is razor-thin.
The ledger jammed mid-tally. The lesson continues without it — everything you have already done is safe.