Not to be picky, but that’s not exactly what it means. It means that the flood plain is expected to flood as the result of a ‘100-year storm’. A 100-year storm event is one that occurs, on average, every 100 years. It could be 200 years between events, or 2 years, but is expected to average every 100 years. You can also say that a 100-year storm has a 1% chance of occurring in any given year. Governments generally set a certain 24-hour rainfall total that would correspond to what a 100-year storm event is. Local municipalities usually do the same for 2-, 5-, 10-, 25-, 50-, and maybe 500-year storm events.
You're right.
Actually I was oversimplifying it...