Whipping egg whites unfolds their proteins and traps air in a network of bubbles. That foam is what gives meringues, soufflés and sponge cakes their lift.
A clean, grease-free bowl matters because fat interferes with the foam. A pinch of acid, like cream of tartar, helps stabilise it.
The takeaway
Understanding the foam helps you troubleshoot: greasy bowls and stray yolk are the usual culprits behind flat whites.