For triathletes, maintaining a high cadence (90 rpm or more) on the bike can help reduce muscular loads and keep our legs fresh for the upcoming run. The piece of equipment that largely affects our cadence is the rear cassette.
Rear cassettes, or the cluster of gears at the rear of the bike, are named according to the number of teeth on the smallest cog and the largest cog. In other words, an 11-23, has 11 teeth on the smallest cog and 23 teeth on the largest cog. The cogs in between have some number of teeth between those two extremes.
When you select a gear on a high-toothed cog, it is easier to pedal and maintain a high cadence. When we pedal on hilly terrain, it is advantageous to have a cassette with a cog that has a high number of teeth. On flat terrain, you can get by with less number of teeth.
You can own multiple cassettes and simply use the one that is appropriate for the terrain in your race. Most bike come with an 11-21 or 11-23 rear cassette. For most triathletes, this size of a cassette is too small. A 12-25 or a 12-27 is a more versatile cassette that can be used both on flat courses and hilly courses.