Motivation is something which makes you achieve your goal. It is subjective.
Motivation is a driving force to achieve your goal, it can be from within (intrinsic) or from external source (extrinsic) or combination of both.
It varies from person to person. It is subjective.
When you use this as a tag, it can mean that you are looking for/suggesting that the post to would be motivational for you or whoever would be reading or you expecting some motivation from people here.