 is a model of propan-2-ol. If the functional group was on the end carbon, and therefore we would name this propan-1-ol, then being on an end carbon would make it a primary alcohol. We have one, two, three carbons, which is why it's propan-ol, and so therefore propan-1-ol telling us that the OH group is on an end carbon. But for a secondary alcohol, the hydroxyl group is located on a central carbon, or one at least that is not on the end, not a terminal carbon. So therefore we have our OH group attached to a carbon, which is also attached to one, two other carbons. The fact that it's attached to these other carbons is what makes it a secondary alcohol.