Restoring a backup with SSH

mysql -uUSERNAME -p DBNAME < /PATH/TO/DUMP.SQL

USERNAME - this username which you use to access MySQL.

DBNAME - the name of the database used for MyBB.

/PATH/TO/DUMP.SQL - this is the path to the backup file that you have on the server. You will have to upload it to the server since you used phpMyAdmin to back it up.