This is a new concept for the older expansions, introduced later in WoW's lifecycle. Basically you pay $40 (or pony up for the deluxe edition) and you can instantly boost an existing or a new character to 58, which ensures that they're “BC ready.” This is what happens after that process.Jun 11, 2021
Are WoW boosting services legal?
Selling in-game items and services such as carries or boosting for real money is not allowed. Selling items and services for gold is allowed but can only be advertised in-game through the Trade chat channel. ... Accounts that are used for the primary purpose of advertising may be closed.