These are forklifts that are designed specifically so they can fit down very narrow aisles in a warehouse. This gives you a couple of benefits: increased space to store things is the key one. You can have smaller aisles and still be able to fit a forklift through them. The other one is maneuverability - you will be able to move a narrow aisle forklift even without much space because they have eliminated most of the stuff that hinders movement and have squished up the main body of the forklift in the design.
The downsides: these are really slow compared to regular forklifts. If you need to move things across large distances, they just will not cut it in terms of speed. That's a problem that can be solved if you have a regular forklift, however, by using the narrow aisle forklift to move the load to a central area and then handing it off to a regular forklift to take it to bulk of the distance. They also usually can't lift as much weight, so they aren't going to be effective outside of smaller loads.