Much of what you do in Java is to define classes that package data and functionality together by concept to represent the desired problem-space element. When you instantiate a class, you create an object that has it's own piece of memory made up of other objects. Java has a peculiar means of manipulating these elements in memory.
Object Oriented programming (OOP) is a programming methodology whose progress of abstraction allows for the conception of elements within the problem space to exist as objects within the solution space. This seemingly one-on-one mapping provides an ideal abstraction of the real world or problem space; consider if you will, the normal objects with which you interact with everyday for instance a maple tree, a bicycle, a car etc..