The best way to understand is practice. Try to download MySQL and execute in it the commands of the lessons. If you have trouble understanding the terms used, you can choose "Glossary" when you can find the definitions of the technical terms of the course.
Good luck, and don't lose hope 😉
If I get it right, you are not understanding the sql lesson. Teach yourself SQL in 21 days helped me learn sql. You can google it and download the free pdf. But like @I'm a Learner stressed, code more....:-)
did you first learn some basic concepts of DB ?
if you just directly read sql without basic DB concepts like foreignkey , one to one and many to many relation and so on.
hope useful :
complete the sql course. if you still dont understand things thats fine try looking at the docs and downloading and testing commands out in sqlite3 at sqlite.org and check out an app on android called “sqlite manager” that has a gui to do things but shows the sql commands being run and even allows typing them instead of using buttons
and feel free to ask questions here and at sql/sqlite wikis people will be glad to help