Pre-emphasis is used in FM transmissions to boost the higher frequency spectrum of the audio modulating signal. This is reversed through de-emphasis in the FM receiver. This is done to reduce noise level that would otherwise be present in the audio signal due to the triangular spectral distribution of noise in FM.
The 75 µs figure simply refers to the RC time constant (tau) used in the high pass filter to boost the high frequency content of the audio modulation in the transmitter. In US broadcast transmissions, tau is equal to 75 µs but it is 50 µs in most other regions of the world.
$$ \tau = R \times C $$
For example, here is an RC circuit that provides 75 µs of pre-emphasis in the transmitter:
simulate this circuit – Schematic created using CircuitLab
The actual RC values would be chosen to match the impedance of the audio amplifier chain. An RC filter with a tau of 75 µs has a cut-off or breakpoint of ~2.1 kHz.