To add to this...
Most routers these days have 2.4GHz and 5GHz frequency channels. You can set up an SSID on each, giving you a choice of the two on your laptop, tablet, TV, etc., when joining a WiFi network.
The 2.4GHz channel is more powerful. It goes longer distance, through walls better, etc. But it is more easily congested if you have a few devices connected to its WiFi. The 5GHz channel doesn't congest so easily, but you get less range from the router.
Also, the weaker the signal (distance from the router, walls in between, etc.), the slower the Internet connection will be.
A WiFi range extender won't really help much. It will allow you to connect further away from the base station, but comes at the cost of much slower bandwidth. Unless you connect the extender to the base station via ethernet.
Ideally, you move the WiFi router to a more central location in the house where all the devices have the best possible signal.
Or more ideally, use wired ethernet connections everywhere possible. I use ethernet for my home theater and my desktop computer where I do my job, and for the time machine backup device (so I get backups at 1GB speed).