TLDR
mysql -u your_mysql_user -p your_mysql_database < your_mysql_script.sql
It’s simply telling mysql:
- which user is doing the database import using -u
- to which database it should be imported using -p
- and finally from which file using <
In depth
Sorry, if you want to go in depth just google it:
https://www.google.com/search?q=import+database+mysql+command+line&oq=import+database