Is there a dot missing from the IP address you quoted?
The issue of "reserving" the address is to make sure you can always access the Humax on your network at the same address. I don't know the Foxsat, but I presume it has options to request an address from the router at boot time (DHCP) or to set an address manually. A DHCP address can be different each session, although typically the router will assign the same address each time, particularly if the connection is used frequently. The router can be configured to reserve the address for the Foxsat (identified by the unique MAC address), but detailed instructions depend on which router you have (usually via a control panel web page).
Manual address setting at the Foxsat carries a small risk of address conflicts unless you are careful not to set any address that is or could be in use by another device, but ensures the Foxsat is always at a predefined IP address. The perfect configuration is to either manually set an address which is not in the router's DHCP pool (the range of addresses the router can hand out to devices requesting an address), or manually freeze the address previously configured by DHCP and then reserved at the router. Again, the DHCP pool is router-specific.
DHCP is useful for devices to connect to your network on a casual basis, without the user being terribly aware of what's going on. The device needs an address; to get access to the Internet the particular address doesn't matter much and the user doesn't need to know it. Specific addresses only become important when you have local resources on your network - eg a NAS, or in this case the Foxsat. Local resources should have fixed addresses so another device knows where to find them (although Windows etc will attempt to locate resources by name rather than address).
By assigning an address outside the DHCP pool, the administrator can impose a logical scheme to fixed resources. For example, on my network I have three HDR-FOXes and three HD-FOXes. All the HD-FOXes need to locate all the HDR-FOXes, and each HDR-FOX needs to locate the other HDR-FOXes... and I need PC web browser access to all of them. Thus they are fixed resources, and their addresses are assigned 192.168.1.11, .12, .13, .21, .22, .23 respectively (my DHCP pool is .64 to .253).
Manual addresses are also useful for overcoming problems with networks which only "wake up" slowly, eg networks based on HomePlugs with sleep mode. In these cases the network connection is not established immediately the device boots, so cannot respond to the DHCP request and may leave the device unavailable on the network. If the IP address is assigned manually, the device will be able to communicate once the network has stabilised.
I hope that helps. For explanation of terms see my
Glossary (click).