br0 allows you to bridge onto the network adapter that has bridging turned on.

Allowing it to directly connect to the network as if it had its own NIC.

How does this differ from the Bridge option that's available in addition to Custom: br0 for containers?

IIRC, the br0 docker network is a macvlan network. The container's virtual NIC will behave as if it's directly connected to your physical network. IOW, it will obtain an IP address directly from your router.

A docker bridge network allows multiple containers on that network to communicate with each other. IOW, the containers are bridged together. The confusing bit is that this bridge network is typically on a different subnet than your physical network. You can see this by inspecting the docker network. Notice how this bridge network is on subnet 172.17.0.0/16.

root@thebox:~# docker network inspect bridge
[
    {
        "Name": "bridge",
        "Id": "49bbd6ae1642a39b678a8db5ad383f10c0e1dac139c4e1244be840ae98bdf6d7",
        "Created": "2021-09-11T07:44:04.07015784-07:00",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "172.17.0.0/16",
                    "Gateway": "172.17.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {
            "com.docker.network.bridge.default_bridge": "true",
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true",
            "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0",
            "com.docker.network.bridge.name": "docker0",
            "com.docker.network.driver.mtu": "1500"
        },
        "Labels": {}
    }
]

The docker host (unRAID in this case) will usually NAT traffic between the physical and bridge network.

The trick is to go to settings > docker then disable docker. Enable Host access to custom networks and then enable docker again. Now your docker instances will be able to access the host on the network.