The single and double stack is in reference to the ammunition orientation in the magazine itself. Since double stack magazines are wider than single stack magazines, a double stack pouch will be needed to hold the extra width.
Double stack magazines are usually found on Glock and newer Springfield pistols that hold a higher number of rounds. Single stack would include almost all traditional 1911 models as well as compact pistols designed for carry, usually carrying 6, 8 or 10 rounds total.