What is object oriented programming really all about?

its a programming way that enables you to define new classes( new type) that you can create instances(objects) from them and instances of the same class share the same attributes and methods.
the use of methods will reduce number of code lines and that will lead to high performance and better maintenance

It's all about better code re-usability, and better code maintainability.
When you have less code in your application, the application is for sure more maintainable, and less complex.
this is very useful consideration for applications that have a life cycle of several years of development, and it's for sure a success factor to minimize cost and error, since simpler applications requires less efforts and resources for the development cycle.
Sometimes OOP comes with a little overhead on performance than procedural implementation.

from all the answers , i think the most accurate is : better code re-usability, and better code maintainability.

it is a programming paradigm that represents concepts as "objects" that have data fields (attributes that describe the object) and associated procedures known as methods.
Objects, which are instances of classes, are used to interact with one another to design applications and computer programs in a simple, maintainable and easy to understand way.

it helps you to add classes , methods and public functions and private and helps you to enhretence between classes , and easy to maintain

its all about classes , objects , function private protected allows enhartence and new teq.
its going to MVC now

It's the ability to create objects of a certain class these objects of same class have same features and specifications this makes the code reusable and easier to maintain, as when you need to modify these objects you only need to refelct your change on the class itself.

OPP is the philosophy of code design , its make the code easier and well documanted anyone can see it after you can understand the code

Modularization Abstraction -- Understandability Encapsulation -- Information Hiding Composability -- Structured Design Hierarchy Continuity

For me I consider it a way of organizing your code, and you can reuse it later easily.
Compartmentalization of your code.
This is my simplest answer for you.
but off course , it has many other things.

strategy behind using oop concept is (object) and its principles inheritance,polymorphism,encapsulation,abstraction every principle used for its own action and advantage.
like inheritance code re usability Abstraction is used with interface for different layers in programming polymorphism is used for maintenance encapsulation is used for hiding information object which define states and behaviours.