
Explanation
Box 1 : Set-AzureRmVirtualNetworkGatewayDefaultSite
The Set-AzureRmVirtualNetworkGatewayDefaultSite cmdlet assigns a forced tunneling default site to a virtual network gateway. Forced tunneling provides a way for you to redirect Internet-bound traffic from Azure virtual machines to your on-premises network; this enables you to inspect and audit traffic before releasing it. Forced tunneling is carried out by using a virtual private network (VPN) tunnel; this tunnel requires a default site, a local gateway where all the Azure Internet-bound traffic is redirected. Set-AzureRmVirtualNetworkGatewayDefaultSite provides a way to change the default site assigned to a gateway.
Box 2 : 0.0.0.0/0
Forced tunneling must be associated with a VNet that has a route-based VPN gateway. You need to set a
"default site" among the cross-premises local sites connected to the virtual network. Also, the on-premises VPN device must be configured using 0.0.0.0/0 as traffic selectors.

Forced Tunneling:
The following diagram illustrates how forced tunneling works

Reference:
https://docs.microsoft.com/en-us/powershell/module/azurerm.network/set-azurermvirtualnetworkgatewaydefault
https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-forced-tunneling-rm