Many people do not think much about gear ratios before buying a bike but they can make a huge difference in how the bike works for you.
Gear range is really important. Often you will see numbers like 32-90 gear inches. This is really important, as to where and how you are going to ride. What gear inches refers to is how far the bicycle travels per pedal revolution. So a bike with 32-90 gearing would go 32 inches for every pedal stroke in low gear and 90 inches per pedal stroke in high gear. My personal preference is I want at least 90 gear inches on a 20” wheel that I am riding on pavement. For me the high gear is most important but I will get as low of a gear as I can with the shifting system as long as I can have my big gear.
If you are just cruising on your bike you will want a low gear to easily climb hills and a fairly high gear so you don't have to pedal very fast when just rolling along.
If you are looking to get a workout on your folding bicycle you are going to want gears that allow you to go faster. I would recommend at least a 90 inch high gear and a 92 or 94 would even be better. Normally you will have to get a 20 inch wheeled bike with
If you are planning on adding racks to carry a load and tour on your bike you will want pretty low gearing so you will be able to make it up hills. A load of even 20 pounds makes hill climbing much more of a challenge. If this is your use your high gear will not need to be as high. An 84 high gear will probably be fine if you are loaded touring.
By changing the Front Chainring on your bike you can significantly change the gearing on your bike. With a 20" 406 wheel my preferred top gear for a workout is a 52-56 chainring with an 11 tooth in the rear. To get the 11 tooth gear in the rear you will need to have a wheel with a Cassette hub.