Classes and objects
One reason for this is that object-oriented programming very much fits the way we perceive and think about the world, at least in the occidental culture. Our way of thinking was, historically, very much influenced by Plato, and philosophers who followed him:
We somehow tend to think about the world as if there are abstract "ideas" (or concepts) and concrete "things", which are entities of the ideas, or implementations of the concepts. So, "ideas" are like blueprints for "things".
For example there's the idea of a human being, and then there are actual human beings: you, and me. Our idea of a human being includes the ability to, for example, remember our own name, and tell it to others when asked. We know that we can ask people for their name, and they'll know it.
Now, object-oriented programming languages allow you to describe things exactly this way: In object-oriented programming languages "ideas" are called classes. And actual "things" are called objects.
As a programmer you would define classes, like a class "User" and a class "Tweet", and then create actual instances of these classes: an actual concrete user with a name, email address, a password and actual tweets. You would also define that users have the ability to remember their own name, and the ability to tell it to others when asked.
Classes are like ideas, objects are concrete things, manifestations of their ideas.
Every object-oriented programming language comes with some "ideas", that is, classes, already baked in, and so does Ruby. For example Ruby has classes for numbers, strings (text), and other useful things. You therefore can, without any further effort, do number calculations, or text transformations.
In your first excercises you will do just that. Later on you will learn to define your own "ideas" as classes, and then use them by the way of creating actual instances or objects, from these classes, and then make them interact with each other and do interesting stuff.
By the way, here is a quote from a book titled The Early History Of Smalltalk from 1993. It has been written by Alan C. Kay, one of the designers of Smalltalk, which was created in the 1970s, and can be considered the first fully object oriented programming language. We think it's pretty interesting to read.
"[Smalltalk's] way of making objects is quite Platonic in that some of them act as […] Ideas from which manifestations can be created. That the Ideas are themselves manifestations (of the Idea-Idea) and that the Idea-Idea is a-kind-of Manifestation-Idea – which is a-kind-of itself, so that the system is completely self-describing – would have been appreciated by Plato as an extremely practical joke". — Alan C. Kay
Anyhow, you can take away from all of this that those universes that we create when we write programs will be populated by objects ("things"), and their characteristics are defined by classes ("types of things", or "ideas").
This will start to make a lot more sense to you once you write your own classes.
There is, today, some debate about the paradigm of object-oriented programming, and whether we should move on to another one, called "functional programming". You might hear about this if you talk to developers. Some functional languages are Haskell, Go, and Clojure, and they have recently started gaining some traction. Don't let yourself be distracted by this too much, unless you really want to. Ruby is a very good language to start learning programming for lots of reasons.