The notes that brass players play are based on the harmonic series. The lowest note is called the fundamental, and it is the same as the fundamental frequency. When the brass player plays the first harmonic, they are producing a note that is twice the frequency of the fundamental. The second harmonic is three times the frequency of the fundamental, and so on.
The harmonic series is also used by string players and woodwind players. However, brass players use the harmonic series more extensively than other musicians because it is the only way they can produce the highest notes on their instruments.