Definition of Software Engineering:
Definition by Standard Glossary of Software Engineering Terminology
"The application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software."
If you like reading a book, I recommend "Head First Object-Oriented Analysis and Design by Brett D. McLaughlin, Gary Pollice, Dave West".
This book covers object-oriented design principles and practice. It is 600 of the easiest textbook pages you will ever read; the concepts are not deep and the book hits every point from many angles.
Whether or not reading the optional textbook, you must follow the readings posted on the website.