Sample rate
How many times each second a sound is measured to store it, often 44,100 or 48,000.
See it in Audio visualizer. Press Microphone: your browser measures the sound tens of thousands of times a second.
In plain words
A computer cannot store a smooth wave, so it measures the wave at tiny, even gaps. Each measurement is a sample. The sample rate is how many are taken each second. Music is usually 44,100 or 48,000 a second. To capture a tone, you need at least two samples for every swing of it.