Shear center is defined as the point on the beam section where load is
applied and no twisting is produced.
At shear center, resultant of internal forces passes. On symmetrical sections, shear center is the center of gravity of that section.
In unsymmetrical sections, if the external applied forces act through the centroid of the section, then in addition to bending, twisting is also produced. To avoid twisting, and cause only bending, it is necessary for the forces to act through the particular point, which may not coincide with the centroid. The position of the this point is a function only of the geometry of the beam section. It is termed as shear center.
Such sections in which there is a sliding problem, we place loads at the shear center.
Reference : Mechanics of Engineering Materials by P.P.Benham