Hearing whether one note is higher or lower than another is the first ear-training skill; every other listening skill is built on it.
Higher means faster vibration; lower means slower. A flute lives high, a bass lives low, and a piano covers most of the usable range in between.
This judgement gets genuinely hard when two notes are close together, one semitone apart, or when the timbres differ. Training narrows the gap you can reliably hear.
Start with big gaps and shrink them. Tavi La Si begins with wide jumps and closes in until you can call one-semitone differences with confidence.
Common questions
Why do I confuse higher and lower notes?
Timbre and loudness interfere with pitch judgement, and small distances are genuinely hard. It is a trainable skill, not a talent; most learners become reliable within days of short practice.