Volume is just the number of bitcoins (or shares, or whatever) that have changed hands in a given time period. On paper, anyway. If I sell you a bitcoin and then buy it back for the same price, that's BTC2 of volume (BTC1 for each trade), even though there's no net change to our position. So be aware that volume can easily be manipulated.

I recommend Economics in One Lesson by Henry Hazlitt. You can download it here (PDF, 1.3 MB).