Imagine you are trying to talk to a friend, and you are standing in a crowded room with lots of other people, all trying to make sure their conversation is heard.. As a result, you have to constantly repeat yourself so the others person ger all the info you are giving them.

Now imagine you and your friend move to a different room where noone else are standing. You can say things one time and the info will immediately be understood.

If your wifi network is using a channel that is occupied by lots of other devices, your wifi will have to use alot of the 'bandwidth' to make sure the other device have all the data and that it is correct, thereby potentially reducing the max speed of your wifi connection. By switching the channel to one where there are less other devices (or maybe no other devices), the data flows better and you can end up with faster and more stable connection.

(this explanation is simplified, and I might not be using 100% correct names etc, because english is not my native language)

The app i use is called 'wifi analyzer pro' and I got it from the F-droid appstore (i think it is mentioned already in this thread), but there are lots of alternative wifi analyzer apps in the normal appstore you could try if you don't feel comfortable installing an alternative appstore (it doesn't replace the normal appstore).

Note: some newer wifi routers will have built in functions to automatically select what it deems the best channel to use, meaning you might not have to change anything.