You've got a 42T chainring (the gear in front). You've got a 13-34T rear cassette (the gears in back). The shifter will have 1st gear as the 34T gear in back. 7th gear is the 13T gear in back. So. You started up the hill in a gear that worked on the flat. It got hard to spin. You downshifted a gear. Spin some more. It gets hard. Downshift. Repeat as needed. Bing! You made it up the hill.

On a bike with more gears, you would need to change gears in both the front *and* the back for some hills. This can be... exciting... Managed to drop my chain that way this week. Good thing I was on the way to the shop for a tuneup anyway. Normally it's not exciting, but if the bike doesn't get normal maintenance, it will be.

If you watch yourself when you ride, you'll probably find you have a favorite gear for cruising around (mine is 2/5, yours is probably 4). You'll probably only hit 7th gear pedaling down hills, and you'll mostly hit 1st gear going up them.