An interferer is any device operating on the 2.4GHz or 5GHz frequency Band.  A device(s) generating radio frequency noise can cause the network to slow down or network disconnects. 

Interference Sources Include:

• Microwaves — the closer the router is to microwave the more network interference you can expect to occur when the microwave is in action. That is especially true for older wireless routers that just like microwaves operate in the 2.4 GHz spectrum.
• Cordless Phones — These phones can cause large signal interference. Wi-Fi Interference will occur during active call time.
• Poorly Wired Satellite Dish — if a satellite dish is not properly wired or the old wires are deteriorating it can cause interference.
• Other Wireless Devices — any wireless device can technically be the reason for a signal interference. These can be wireless speakers, baby monitors, garage door openers, etc. Some other wireless devices operating in 2.4 GHz or 5 GHz spectrum, including microwave transmitters, wireless cameras can also contribute to wireless interference.
• Power sources — electrical railroad tracks or power lines that are in close proximity can be causing WiFi interference. If you can try not to position your Wi-Fi router near power lines in the wall or close to the breaker box.
• Poorly shielded cables — if you suspect that a certain device is the cause of interference, try to disconnect it and use a different cable for it.
• Some external monitors and LCD displays — these can be specifically annoying on the 2.4GHz band between channels 11 and 14. If you are working with a monitor connected to a closed laptop the interference might be even stronger. Make your access point use 5 GHz or a lower 2.4 GHz channel.
• Neighbors' WiFi — powerful WiFi networks that overlap can affect each other.