It is just as it states-one shank is 'higher' than the other. If the shank (area on the foot from the top of the pressure foot to the spot that has the slot for the screw that holds it on the machine) is around a 1/2 inch (from memory that is the measurement) that is a 'low shank'. Any thing longer is a 'high' shank. Once you see the diffrence you will be able to pick them out on sight. Also, Singer had several 'slant shank' machines. That is also very easy to spot. The shank 'leans' to the back when on the machine. These are interchangable.... but you can use most all low shanks on low shank machines and vice versa. The biggest thing you have to watch is if the foot covers the feed dogs. If you can see the feed dogs when the foot is down it won't feed the material right.