Its a physical line test made with a specialized meter. It can be very effective for troubleshooting.
Might be tech heavy for you but this is a good overview.
Its really great for checking physical issues with lines and IMHO it should be run on EVERY new connection, after the second visit (for same issue), or after 48 hours of no sync (depending on history).
It is almost never 100% clear but a good tech can interpret potential issues to see if they would be an effective fix to improve the signal.
Its just an expense though most ISPs don't go through unless necessary.
You can request one usually from you ISP (who passes it onto bell) of if your on their lines they can do one for you. Just takes a bit of pressure and sometimes an escalation.
Some techs you may be able to ask them to do one (if they have it on the truck) though it could be a voluntary charge.
(again though its worth it, they are far more to rent and use DIY)