The OCaml module system is based on structures and signatures. Structures are the core of the module system; in fact, we've been using them all along without thinking too much about them. Signatures are the types of structures.
It's tempting to make an analogy to object-oriented languages you might know. For example, we might say that structures are like Java classes, and that signatures are like interfaces. Or, we might say that structures are like objects, and that signatures are like classes. Both analogies are imperfect. So, to begin, it might be best to put any analogies out of your mind, and just treat OCaml modules as a completely new idea.