A mapping table is a table of always 2 fields and the table only exists during the reload procedure. It is used when you want to lookup a value.
It's used a lot when you load a fact table and want to lookup a surgate dimension key from a dimension table. In this case you create a mapping table with the unique business key and the corresponding surrogate key.
Mapping load is faster than the lookup function, but the lookup function is more flexible.
You should take a look in the help area, and search for mapping load and lookup. I believe there are some examples.