Okay kiddo, let me explain hydroxyl value to you. Hydroxyl value is a measure of how many hydroxyl groups there are in a certain substance like a chemical or a material. Hydroxyl groups are made up of hydrogen and oxygen atoms bonded together. They are kind of like little magnets that like to stick to other molecules.
When we say the hydroxyl value of something, we mean how many of these little magnets there are in that thing. This number is important because it can tell us things like how reactive or how soluble the substance is.
So basically, if we want to know how many of these little magnets (hydroxyl groups) are in a certain substance, we check its hydroxyl value. Simple, right?