Shakti is a Force. It is the unseen force by which this whole Universe was created. Nobody knows who created this force and how and when was it created. Vedas have called it anadi for this reason.
Shakti cannot be seen but can be only felt by us or we can simply say that it is aabhas matra and agochar i.e something which is not seen with the eyes.
It is only seen after it manifests itself in some activity or creativity. The Cosmic creation that we see around us is the manifestation of that One Force or Shakti.
Shakti is sanatan which means eternal and it is limitless and unending hence called anant in Sanskrit. The Force is indestructible hence very powerful and potent.
Shakti is worshiped because it is pure and benign.
The Gods, deities and the saints are the ones who possess this force and hence we worship them. In fact, it is their Shakti that we submit ourselves too.
Shakti serves two purposes- constructive and destructive.
Constructive force is silent and destructive force noisy and loud.
In all of us lies just a fraction of the Cosmic force of which we are unaware. It lies very deep inside us covered by many layers of thoughts and sanskaras. This force lying deep inside can be tapped with the help of deep meditation.