Two firms sell the same good and each chooses only how much to make. The market price falls as total output rises: P = a − b(q₁+q₂). Given whatever the rival is making, a firm's profit-maximising reply is a straight line — its reaction curve, with slope −½.
Let the firms take turns best-replying and their decisions trace a shrinking staircase that spirals onto the point where the two reaction curves cross: the Cournot–Nash equilibrium. There neither firm can earn more by changing output alone. Knock a firm away and best replies drag the market straight back — the intersection is an attractor.
The ghost diagonals mark the benchmarks: were the two a single monopoly they'd restrain output to keep the price high; under perfect competition they'd flood the market until price met cost. Cournot duopoly always lands between the two — more than a monopoly, less than competition.
Something in the simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.