An insurance company takes in money from two sources. Premiums are the obvious one. The second is investment income: premiums collected in earlier years and not yet paid out sit as reserves, and the company invests them, usually in safe and easily sold assets, because it must be able to reach the money when a large claim arrives.
Money leaves in three ways: claims paid to customers, the expenses of running the business, and whatever is left over as profit, or the shortfall as a loss. The chapter's example has 100 drivers whose accidents cost $186,000 in a year, so a premium of $1,860 each covers the claims exactly. The sliders set the premium and the investment income, and the side panel works out whether the company ends the year ahead or behind. A toggle draws each arrow in proportion to the money moving through it.