Thank you for getting in touch with us. These brown lines are usually the result of pigments being deposited right below where the nail is growing. The medical term is melanonychia.
There can be quite a long list of possible causes for this. But most of the time it is not something dangerous.
You should check it out with your GP or skin specialist.
What we would do is to take a close look at the nail and the skin around it with a "dermascope", or a good magnifying glass to advice you further.
Take Care.
