This page allows you to define LAN-to-LAN traffic where it otherwise would be blocked. For example, assume we have two LANs, one primary (LAN0/br0) and one secondary (LAN1/br1). Devices on LAN0 will be able to communicate with devices on LAN1 (and the other way around).
On: Checking this enables the defined rule on this row of the table.
Src: (Logical) Source LAN for the rule on that row of the table.
Src Address: This (optional) field narrows the rule to a specific IP address or set of addresses within the Src interface.
Dst: Here, you specifies the (logical) Destination LAN for the rule on this row of the table.
Dst Address: (optional) narrows the rule to a specific IP address or set of addresses within the Dst interface.
Description: This is a free text field in which to enter whatever you wish as a reminder, note etctera.
Regardless of LAN Access rules, by default a LANx device is able to reach (e.g. ping) all the router's LAN interfaces (only). This is by design.