48 Khz means:
Sample frequency of 48 kHz (48000 Hz) = 48000 samples per second.
While the sample frequency considers the time factor, the bit depth represents the resolution of the single sample.
A resolution of 16 bits means 65,536 possible levels (2high16). With each bit, the resolution doubles, so that with 24 bits, 16,777,216 possible levels are available.
More important than the sampling rate, e.g. 48 Khz, is the dynamic range, which in PCM is described by the word width. It indicates how many volume steps can be digitally encoded.
While an increase of the sample rate from 48 kHz to 96 kHz can hardly be perceived due to the low frequency content in the upper range of the spectrum, an extension of the sample width from 16 to 24 bits is clearly audible to the ear.