It is a craze line. They are extremely common and it's nothing to worry about. Having your teeth cleaned will most likely reveal all sorts of imperfections. Don't worry about them. They are stable and you can't undo them.
But a crack is not painless, it's often not visible, you will know about it usually when you bite down on something hot and it feels like an electric shock has hit the tooth. The best way to protect against real cracks is to not abuse your teeth with food or substances that should not be chewed. For example avoid chewing boiled sweets, be careful of popcorn kernels that have not popped, do not use your teeth as a tool for holding pens and other objects, you have hands for that. Check fish for any bones.
When you do chew do it mindfully and don't just chomp down on food that may contain hard chunks. If you think you are a grinder get your dentist to make you a nightguard. You can control your jaw in the day but at night we all need help.
I cracked a tooth crunching on a mint. Thankfully it was repaired without the need for root treatment or extraction.
Well done on getting your teeth cleaned, but try to relax about the craze line because worrying can make you grind!
