The General Fund is the main pool of money for AAP. Money in this fund can be used for any legitimate purpose of AAP.
This is also the default fund. If you do not specify a fund when donating, this is where the money goes. Because it is the default, this fund usually has a lot of money in it, comparatively. Historically, it is surpassed only by the fund for APPD. Lately, though, the Savings fund is gaining ground.