The number of spools refers to the number of separate work sections a valve may have. So you can have a one piece valve that has one or more spools, or a stackable valve that can have one or more spools. A spool is basically a machined piece of steel (to simplify, looks like a round bar with groves machined into it) that controls the hydraulic fluid flow through the valve. As you move the lever one way it routes oil to the work port which then moves a cylinder, powers a motor, etc. A double acting valve has two work ports, each port to provide power to a cylinder for instance(one for push the other for pull), or to a motor to enable it to be reversed. A single acting valve will have only one work port. Pull the lever one way to route oil to this port to power a single acting cylinder, pull it the other way and it routes the oil from the work port to the out port. I hope this helps.
