An object is a representation of a specific idea. In Construct 2 this definition still applies. Mentally, when you think of a specific idea you may think of something like a book or a ball. I'm sure you can appreciate that although you know what a book looks like, you also know that there are many unique books.
Although you can have many instances of a type of object, no two objects are the same in Construct 2. For example, you can have 30 Book objects in a single layout, however each book will have be different. This is clear, because every instance of any object in Construct 2 is assigned a unique value known as a UID.
Hope I helped!