It is pretty simple that you cannot use root user in 5.7 anymore without becoming a sudoer. This signifies that you cannot run mysql -u root anymore and have to do sudo mysql -u root instead.
It also signifies that it won't work if you're using the root user in a GUI. To run it you need to create a new user with the required privileges and use that instead.
If you want to get more insights into SQL, check out this SQL Course from Intellipaat.