I noticed a white line going across my tooth, just above my gum. what could it be?

Decalcification: May be the start of a cavity. Why don't you have a local dentist take a look?

Thin white line?: A thin white line on tooth just above the gums could be de-mineralization caused by prolonged plague buildup. It could also be calcified deposits that can be scrapped away. A photo / visual exam will be needed to complete the answer to your question.

Early stage of decay: This is where mineral is removed from the tooth surface near the gum line. At this stage, early intervention with topical fluoride products can stop the process. See your dentist ASAP.

3: 3 possibilities come to mind. Horizontal fracture, accumulation of tarter, or decalcification (early stage cavity). Please see your Dentist for differential diagnosis and treatment plan. Thank you for demonstrating just another reason why regular professional care is so important.