it's simple, the source address is IP address or network from which the virtual server accepts traffic, and the destination address specifies destination IP address to which the virtual server sends traffic.
source address is optional, you can leave it empty. The virtual server accepts clients only from one of these IP addresses, and by default F5 accepts all IPs (0.0.0.0/0). if your VS should only be reached from a specific range or IP you can configure it in this section.
Regarding "Destination Address/Mask" this is the listening IP of your VS.
This IP will be propagated via gratiutous arp... to inform the network elements around that this IP is carried by F5 ...