Picture walking that ridge and coming to a spot where the contour lines start getting closer together, signifying a rise in elevation. As you ascend that rise and get to the top,picture this as the back end of a horse's saddle. As you break over the side, [ Dropping down into the seat of that horse saddle ] continue along the seat and you hit another rise in elevation,moving to the top { the front end of the horses saddle } That area, low point, that you just crossed, { the seat } is what is commonly known as a " saddle ".

Keep in mind, it won't always occur on a ridge, but that is where they are prominent/predominate, and are easily spotted when viewing a topo map. These areas are natural travel areas for game, and excellent spots to plan a hunt around.