Very bluntly, it can be related to our hearing range or the range of an instrument or voice. If we say ‘in the upper register’ we mean that it has a high pitch. Low pitch sounds belong in the lower register. Now, if we refer to a specific instrument, the high and low registers are related to the range of said instrument. For instance, violins belong to the upper register. However, their lowest sound, which is in their low register, belongs somewhere around our mid register. It’s not as complex as it sounds. Think of it relatively. Let’s think of numbers 1-10. Number 7 is a big number, isn’t it? But when we think of numbers 1-100, number 7 is a small number, right? It’s as simple as that.