An index can be explained as a sorted list of the items in a register. It is very quick to lookup the position of the item in the register, by looking for it's key in the index. Next the the key in the index is a pointer to the position in the register where the rest of the record can be found.
You can have many indexes on a register, but the more you have, the slower inserting new records will be (because each index needs a new record as well - in a sorted order, which also adds time).
