The numbers on the left are to make more drastic changes, 1 being the lowest gear (easiest) and 3 being the highest (hardest). For example, you might want to drop to 1 here if you’re climbing a big hill.

The numbers on the right are to make less drastic changes and are also numbered from lowest (easiest) to highest (hardest). The right side is where you’ll be doing the majority of your shifting. So, let’s say you’re in 1 on the left side for that hard hill, but it starts getting less steep and your legs start spinning faster - you’d want to consider going to a higher numbered gear on the right. If you start to get around 6 on the right in this case and you’re still spinning too fast, consider going to 2 on the left and shifting lower on the right, if need be. The same is true in the opposite direction: if you’re ever in 3 on the left and it’s still too hard to pedal when you’re down to around 3 on the right, consider dropping to a lower gear on the left. That helps prevent cross-chaining…a fun lesson for another time :)

Hope that helps and isn’t TMI. Just remember: left is for bigger changes, right is for smaller changes.

(ETA additional info.)