Brown spots on my teeth? Moreover, I just had a dentist appointment... They found three cavities, and filled them. I have a thin brown line, less than a quarter inch, on my front tooth- they didn't even mention it

Neonatal line. Thin brown line may be neonatal line, a darker of the day you were born. Brown spots, brown line, may also be from taking tetracycline, or an illness, or a systemic problem, while the teeth were forming. Did you ask the Dentist about the brown spots/lines? that's the best person to ask. Give your Dentist a call.
Professional prophy. That is usually gotten off very easily with professional cleaning, flossing and good brushing technique. Schedule an appointment with your dentist or hygienist to remove stains.