The two numbers that define PCM digital audio — bit depth sets dynamic range, sample rate sets the highest frequency that can be captured.
PCM digital audio is described by two figures. Bit depth (16-bit, 24-bit) sets the dynamic range — the span between the quietest and loudest sounds. CD's 16 bits already gives about 96dB, more than most rooms and recordings use; 24-bit's extra headroom mostly helps during recording and mixing. Sample rate (44.1kHz, 96kHz, 192kHz) sets the highest frequency that can be reproduced — 44.1kHz already covers the full range of human hearing.
This is why "hi-res" (24-bit/96kHz and up) sounds better in theory more often than in practice: a great 16/44.1 master beats a mediocre hi-res one every time. The master matters more than the numbers.