Basically, it’s keeping the intervals of a music phrase intact but shifting the entire thing to a higher or a lower frequency, thus changing its notes. It’s like taking the same route but changing the starting point. I know that it doesn’t really make sense IRL, but it does in music. Think of numbers 1 to 7 as in 1, 2, (…), 7, 1, 2, (…), 7 and so on. If we have the pattern 1-3-5-4 and want to start from 4 instead of 1, we would be transposing it 4 steps higher or 5 steps lower. So, the new pattern would be 4-6-1-7. The notes are different but their intervals remain the same. That means that the melody doesn’t change; we just hear it from a higher or a lower starting point.