They're brown stains. To confirm they are just stains and to have your teeth professionally cleaned, see a local dentist.
Stained Teeth. Good day; Brown stained teeth are usually seen in individuals who drink a fair amount of tea, coffee or red wine. The Tannic acid in tea , the acidity of coffee and both white and red wines are all likely to stain your teeth. Where possible use a straw or attempt to rinse your mouth after drinking these liquids.
Could be. caused by several things. Start with a professional cleaning and polishing and ask the advice of the dentist.
Time for a cleaning. You should have your teeth professionally cleaned twice a year (or more in some cases). If you're not doing this, get to it. If you are already doing this consider whitening or talk with your dentist about cosmetic dentistry. There is always a solution.
Brown Stains. Brown stains are usually caused by surface deposits on the tooth from tea, coffee and smoking. If that's all that it is it can be easily removed with a professional cleaning from you dentist or his/her dental hygienist. When was your last dental check-up and cleaning. You should be going every 6 months. Not seeing your dentist on a regular basis can result in much more serious problems.
Dark stains. More than likely, the dark stains are a result of heavy intake of tea or coffee. A professional cleaning should remove them without too much effort.
External stains? Coffee, tea, wine, tomato sauce, smoking, can all cause browning of your teeth. Some mouth washes with Chlorhexidine can cause brown stains. Antibiotics can cause internal staining. Its worse if you are not having your teeth cleaned regularly, sometimes every 3 months.
See below. Discuss with your dentist for a definitive diagnosis. Smoking, coffee, medication etc. can cause staining which can be lightened with professional bleaching.