A WiFi Mesh Network System is a set of a few WiFi access points (aka routers) that work together as a single WiFi network at home.
Historically, a WiFi network consisted of a single WiFi router that enabled the wireless connection of smartphones, computers, printer and other devices.
However, as Internet speeds evolved, connected devices proliferated with things like Smart TVs or WiFi cameras, and the need to get a wireless connection everywhere in home became a must - a single WiFi router was not always sufficient.
Hence, the WiFi Mesh Network System was invented.
A WiFi Mesh Network System consists of multiple (at least 2) routers:
The primary unit and secondary mesh units may be the same (where only the actual Internet connection distinguish their functionality, as is the case with Google WiFi mesh network system) or different (as is the case with the Ubiquity AmpliFi HD mesh network system).
A WiFi Mesh Network System may be too expensive and even redundant for some homes, where a WiFi Range Extender would perform just as well with at a much lower price point.