Aha, good question.
That's probably the first question any beginner collector would ask. The KM# stands for Krause-Mishler number. All coins are organized by these numbers by type. Slight differences are written KM#__.1 and changes in metal are written KM#__a.
Other catalogue numbers exist too. Y#, C#, and K# from most to least used are used instead of KM# when Krause didn't record the coin.
Use
Numismaster to look up coins not on Numista yet. It will give you the KM# plus additional information.
By the way, welcome to Numista.