A WiFi router (or simply router) is the device that sends out the signal that allows your smartphone, tablet or computer connect to the internet without an internet cable plugged into your device or mobile data turned on.
If you are close to the router the signal strength is higher, and your internet connection will be better. If you are further away from the router you will notice that the signal strength is getting weaker, web-pages might take longer to load or stop loading fully. If you walk away from a router, let’s say you have been at a cafe with great WiFi and you are leaving the cafe to walk somewhere else, your internet connection will get weaker and weaker, until you are disconnected from the WiFi when you are out of reach of the WiFi router’s signal.
To see if you are within reach of a WiFi (which is the same as being within reach of the WiFi router’s signal) you can open your native WiFi manager. The WiFi that appears in the native WiFi manager list are the WiFi you are within reach of.