In the `atrim` filter, what's the difference between `start_pts` and `start_sample` ?

The documentation is unfortunately terse: https://ffmpeg.org/ffmpeg-filters.html#atrim

start_pts
    Same as start, except this option sets the start 
    timestamp in samples instead of seconds.

start_sample
    The number of the first sample that should be output.

What's the difference between the "number of the first sample" and "the start timestamp"?