@Kuriarko 

Here is what the user manual says:
"Guest Network Isolation - If enabled, guests are isolated from each other"

It means if you enable this feature, then your guests' wireless devices will not be able to exchange data between each other directly within the Guest Network. This is fine if your guests merely use your Guest Wi-Fi for accessing the Internet, which is usually the case.

However, if, for example, your guests wanted to copy files between their devices over the Guest Wi-Fi or if you were to connect a printer to your Guest Wi-Fi for your guests to print out stuff or if your guests who are connected to the Guest Wi-Fi wanted to play certain multiplayer video games against each other, then you would need to disable that Guest Isolation feature.