I have noticed the past two weeks when i brush that it feels like am there's a little intention on the side of one of my back teeth. what is it ?

Indentation: It could be as simple as a developmental buccal pit, or it could be a cavity. Really cannot tell without an exam. If a cavity, it needs to be restored by a dentist.

See DDS: See your dentist to determine if this is the normal anatomy of the tooth or if something is fractured and needs to be addressed.

Could be: Could be normal anatomy that you've just not noticed before. More likely a small cavity called a "buccal pit"