Some special ISA cards require this area of memory for them to work properly. Enabling this function reserves the memory area for the card's use. In some cases, it may also prevent the system from accessing memory above 15MB.
If you enable this function, 1MB of RAM (the 15th MB) will be reserved and is therefore not available for the OS' use. So, if you have 128MB of RAM, this feature effectively reduces the usable amount of RAM to 127MB.
In certain motherboards, this feature may actually render all RAM above the 15th MB off-limits to the OS! In this case, you will effectively have only 14MB of RAM, irrespective of how much RAM your system actually has.
Since ISA cards are a thing of the past, you should always disable this feature. Even if you have an ISA card that you absolutely must use, that doesn't mean you must enable this feature. Most ISA cards don't need this reserved memory area. Check to make sure that your ISA card absolutely requires this memory area to work properly before enabling this feature.