access path

["Leif Svalgaard" <leif@xxxxxxxx>]

I have been working on and with this machine for upwards
of two sunspot cycles, and I have a confession to make:
I see references on this list and elsewhere to an "access 
path" to a file. I have studied the internals of this machine, 
I have looked far and wide, but I'm still baffled: what the
h*** is an "access path"? where is it? what does it do?
Why must it be "rebuilt" at times and what does that entail?
Somebody enlighten me...