I just bought a lawn mower a Briggs and Stratton 300 series mower... it has oil and gas, and when I go to pull the pull cord, it only moves about an inch, then it won't budge after that... what is the problem?

If you just bought it, take it back. Problem can be serious or an easy fix.

Serious
If you cannot move the blade by hand. Remove plug first.
Engine damage

Minor
Starter clutch defective. About $15

Yes, if it is new, carry it back.

I know this is most likely solved (being that it was answered in 2008), BUT...

If the push mower is BRAND NEW, out of the box and you can not get the pull cord to move more than 4" after the very first pull, check to make sure there isn't a bar (from the Handle Frame) obstructing the blade.

When the mowers are shipped (or purchased "in-box") they handle frame is about 8" into the base obstructing the blade, making it impossible to pull the cord (the cord spins the blade).

It's an "obvious" mistake, but made frequently by people not reading directions thoroughly :)