What is the concept of soft starter?

A Soft Starter is a power electronics variant on a reduced voltage starter. The key difference is the soft start is gated on and ramped up in voltage until you reach full speed, at which point it can be bypassed to save the energy loss in the power electronics; it thus can be described as a 'bumpless' transition to full speed since you really don't get a momentary inrush 'bump' as the standard reactor, auto-transformer or delta-wye starter gives when it is transitioning from start to run configuration.

You may wish to visit some of the major equipment manufacturers' web sites for further information; they typically have good technical references in their online libraries -- Eaton Electric, Gozuk, Square D, General Electric and Siemens are all good starting points, just make sure you follow down into the motor control area of their product offerings.