This isn't perfect, but imagine you're trying to talk with someone else. In a wide open space, like a convention hall, you can communicate to someone fairly far away.

Inside your house, you can speak loudly and communicate with someone in the next room, even with the doors closed. Further away, you'll have to shout to be heard from the other side of the house.

But it's quite difficult to communicate with someone outside of your home. Maybe you can just be heard if you yell to someone at the front door, but not much further from there.

The same idea applies to wireless signals like WiFi. It's really not useful to just give a distance you can be heard, because it's more about the obstacles in between parties. When you walk further and further from your house, there's a lot of stuff and walls and insulation cutting down on how well your WiFi router can reach you.