How to interpret ShowPlan?

Hi
I'm trying to use Showplan in Microsoft Access.
I see the results of Showplan in the text file when i execute a query
in Access, but I don't know how to interpret the results. For instance
what does it mean that Access "using temporary index"? I'd expect
Access to use not a temporary index but the index that I have created
on the tables.
Does anyone know where one can get more insight into the information
that comes out in the file from Showplan?
Best regards,
Kim