Let’s think of bars and rhythm. Let’s take a bar in four-four. We can count it: 1-2-3-4. Normally, the accent, the emphasis is on 1 and on 3. We can also count a bar in four-four as 1-and-2-and-3-and-4-and. The beat with the most emphasis is 1, then in decreasing order it’s 3, then it’s 2 and 4 and lastly, it’s all the ‘ands’. So by saying ‘off-beat’, we mean that we add notes to the beats that are accented the least. We add interesting stuff to uninteresting beats to make the interesting stuff more interesting.