what is the use of reduced mass??

i mean where is it used(in which cases)and how????

Reduced mass is very useful for solving two body system problems.

Say if two blocks/particles are moving along with some force(like spring) between them then it becomes difficult to calculate energy stored in system as they move with different velocities . But since extarnal force is zero all these problems can be solved in cm frame eg KE=1/2(mu)vrel^2

if say velocity of one block is known you could calculate velocities of both blocks in CM frame and find relative velocity.

Similarly if you know energy you could calculate relative velocity using reduced mass