A guest network is a second wireless name served by the same router, but walled off from your main one. Devices on it can reach the internet and nothing else, which means a visitor's laptop cannot see your shared folders, printers or backups.
The bigger reason to use one is smart gadgets. Cameras, plugs and televisions often run software that is rarely updated, and they sit on the network for years. Putting them on the guest side limits what an outdated device could reach if it were ever compromised.
There is a trade-off. Isolation also blocks the useful things: printing from a guest device, casting to a television, or reaching a network drive. Decide which side each device belongs on based on what it needs to talk to.
Guest networks share the same internet line and the same airspace, so they do not add speed. Heavy use by a visitor still slows everyone, and the router still needs a sensible position to cover the building.