If your bike has manual gear shifting you will need to change the gears to adjust to the terrain you are riding. You can drop into an easier gear when going uphill or a harder gear when you want to go fast on the flat or downhill.
Use your gears so you can maintain a comfortable speed of pedalling, e-bikes work at their optimum around 60-80 rpm ( revolutions per minutes), so a little bit faster than one complete pedal stroke per second. If you pedal very fast, the bike stops giving you the support of pedal-assist, and if you pedal very slowly in a hard gear you get more assistance from your e-bike motor.