Think of a kitchen roll (or loo roll) coming off the wall, then place it on the ground and pull the roll upwards....
You can see the difference in straight wound or cross wound spools if you look at a variegated thread. The straight wound spool has the threads in straight or stacked layers, the cross wound threads make a zig zag pattern.
The two types of spools are designed to have the thread delivered to the machine differently. The straight wound thread needs the spool to rotate to come off the spool correctly, the crosswound off the top of the spool and the spool remain still. This can be achieved by having the straight wound spool upright on a felt or foam pad or a cross wound spool on the horizontal with a spool retainer keeping it stationary. A cone holder can be successfully used as an alternative method with cross wound spools if you do not have a horizontal spindle on your machine.