The three levels of racing that are listed are the most common racing that riders will be exposed to, that is, national, state, and club level.
Club level racing is usually referred to as your local crit or road race, club races are usually held every week year-round, they require an official Cycling Australia license. The race entry cost is cheap ($10-$20 mostly). They are a great way to get super fit, have fun, and also mix it up with some of the faster rides in your city.
State-level racing involves any racing that is run at the state level by your cycling organization. Cycling Queensland, Cycling NSW, etc. State races usually hold higher competition then club racing as more people from around the state will prioritize and train for these events.
Next up we have national-level events, these are the top tier events in the country, national-level events are held by Cycling Australia. These events have the highest level of competition in the country, every year the NRS (national road series) is held alongside the National championships in Ballarat.
For the beginner racer, we would advise you to spend at least a few months racing local club level crits and road races until you’ve worked your way up to at least C grade. This will give you the group skills, fitness, and confidence before you move onto bigger, more competitive races.