Foot volume is the amount of vertical space your foot takes up. This can be at any point along the foot, including at the arch or instep, the ball of the foot, or the toes. If your foot is "tall" in any or all of those areas it is high volume. If your foot is low to the ground or shallow, then it is low volume. This impacts shoe fit significantly, since some brands cater to low volume feet, others to high, and most to a middle ground. If you have high volume feet you may find that many shoes feel tight over the top of your foot. If you have low volume feet than you may find that shoes often slip around and you can't get them tight enough over the top of your foot. Read this article for more information on foot volume + shoe options.
And refer to the Fit Hacks For High and Low Volume Feet in this FAQ for a few tricks.