it is the physical theory published n 1905 by albert  einstein . it replaced newtonian notions of space and time and incorporated electromagnetism as represented by maxwells equation . the theory is called " special " because it is a special case of einstein's principle of relativity where the effects of gravity can be ignored . tne years later einstein published the theory of general relativity which incorporated gravitation .
postulates  :

1 . first postulate ( principle of relativity )
every law of physics should look same mathematicaly to every inertial observer . the laws of physics are same regardless of the inertyial frame of referance.

2.  second postulate ( invariance of c )

the speed of light in vacuum , commonly denoted by c , is the same to all inertial observers , is the same in all directions , and does not depend on the velocity of the object emitting the light .

         special relativity has severall consequences that struk several people as bizarre . for example , the formulas for energy implplies that when an object is at rest ( v = 0 , gamma = 1 ) , there is non zero energy remaining .. : e = mc^2 . this energy is refferd to as rest energy . taking this formula as face value , we see that in relativity, mass is another form of energy !! the impliocations of this formula on the 20th century has made it one of the most famous equations in all of science .

in SR there are 2 different notions of mass . one of them is invariant mass or rest mass  m0 . another definition of mass is givn by relativistic mass which is given by m = gamma * m0 ( gamma = 1/(root 1 - (v/c)^2  )  ) . the invariant mass is an importanmt quantity in the field of general relativity and quantum field theory . thus many physicists simply refer to the mass when they actually mean invariant mass , while they refar to relativistic energy instead of relativistic mass .
this theory also stated that it is not possible for matter to have higher velocity than velocity of light in vacuum .