A certified course is a race route that's been independently measured and verified to be an accurate distance, using a standard set by an athletics federation. Certification confirms the course isn't short, which matters for record eligibility and personal best comparisons. It's usually noted on the race website with a certificate number.
It means a qualified measurer has verified the route is at least the advertised distance, using a standardized measuring method. This is usually done by a national athletics federation or World Athletics.
A certified time is eligible for record purposes and accepted for entry into other races that require a qualifying time. An uncertified course might be slightly short, making finish times unreliable for comparison.
Race websites usually list a certification number or badge from the relevant athletics federation. If it's not listed, it's worth emailing the organizer directly before relying on the result.
Yes, certifiers deliberately measure courses to be a touch longer than the exact distance, never shorter. That's why your GPS watch often reads slightly over 42.2 km on marathon day.
No, plenty of well-run marathons aren't certified, especially smaller or newer events. It doesn't make the race less enjoyable, it just means the time may not count for qualifying purposes elsewhere.