What "sailing the course" means is now entirely in the definition sail the course: start, leave every mark on the required side in the right order (the string test), touch rounding marks, go through gates from the right direction, then finish. Rule 28 simply says you must do it, and that you can go back and fix a mistake as long as you have not finished.
A boat that does not sail the course is scored NSC by the race committee without a hearing (rule A5.1).
Common misreadings
"I can fix a missed mark after crossing the finish line"
Once you have finished you cannot correct. Note that by the definition finish, a boat that continues to sail the course after crossing the line has not finished, so if you realise at the line that you missed a mark, keep going and fix it.
Changed in 2025
The details that used to be in rule 28.1 (starting, finishing, which marks have a required side) moved into the definition Sail the Course.