What could the brown spot on front tooth be?

Many things: Just stain, tooth decay, re-calcified old decay etc, etc try to see a dentist and they will tell you after they examine.

Possibly a cavity: This brown spot could be a cavity forming on your tooth, or it could be a spot of stain. If it is a cavity, it should be treated as soon as possible as it is undermining the enamel of your tooth. If it is just stain, getting a cleaning will remove it. The best course of action is to see your dentist as soon as possible so that it can be determined what the brown spot is.