Nothing nefarious ...
The Guest network is a special 2.4Ghz network that is supported by your router if you want to turn it on (enable it) will allow you access to special feature:
This network allows you to grant "Guests" who may be visiting your home access to your wifi network without needing to give them your main Wi-Fi network password (you set a different password on this SSID than for your main network).
You can enable/disable this network based on if you have Guests in your home who you want to give access to.
In addition, this special network allows them to get to the Internet but NOT to see any of the devices on the rest of your network (specifically the other Wi-Fi networks) which protects you from any virus or other malware that may be on their devices and also keeps any nosey guests from poking around your network.
Business customers (such as maybe your local coffee shop when you were actually allowed to go inside and sit down for a cup of coffee) can use this provide customers access to the internet (free wi-fi) while maintaining a separate network for their business systems.
You can change the name of the network if you like to something less cryptic or to match whatever theme may like for your home wifi network names.
Also, some families put the "kids" on this network and don't give them the password to the main wifi network. By doing so, they can enable/disable. this network to turn access on/off to the Internet during certain hours (such as until homework is done or late at night when they should be in bed).