There are a lot of great tutorials on understanding WiFi so I won’t try to recreate them here, but you can think of WiFi channels like FM radio stations. When you’re between coverage areas sometimes you can hear two stations at once because they’re broadcasting on the same frequency. In the same way, two WiFi networks broadcasting on the same channel cause interference that degrades signal quality.
For FM radio in the US we have the FCC that licenses the frequency and makes sure no two FM stations are using the same frequency in the same area, but for WiFi it’s up to you (and your neighbors) to pick a channel no one else is using.
As @Wrecks0 mentioned, most modern routers will scan for other networks and try to pick the channel with the least interference, but that may be a moving target and only accounts for networks near the router, not devices on the edge of your home. Thus it’s often advisable to scan for nearby networks yourself and determine which channel is the least congested. I use Android apps like “WiFi Analyzer” or “WiFiman” to scan for networks. Most will display a graph showing WiFi channels 1, 6, and 11 (for 2.4GHz) with the nearby networks centered over the channel and signal strength indicated by how tall the graph is:
You’ll want to check in several different locations around your home, perhaps next to a device that has poor signal quality, and try to pick the channel with the fewest networks and/or weakest signal. Using the example graph above channel 1 is probably the best choice.
In summary, the best thing you can do to stabilize your WiFi is to turn off “auto channel” on your router and pick the best channel manually. That will ensure that your network is not changing and allow you to focus on optimizing other variables like physical placement of your router (central location, mounted as high as possible).