Question

what is meant by space time? please explain briefly and its uses.

Answer 1

Space time is time combined with the 3 dimensions this makes it the 4th  dimension.

Answer 2

space time : 4-dimensional real space with X, Y, Z axes and time dimension.

This is used for denoting the position of a particle at time t in space.
      like r = f(x,y,z,t) = position vector

In spherical coordinates   r = f(theta, fi, t)

This is used for studying particle motion, displacement, crystal behaviour, space positioning.   The time varying behaviour of objects can be studied.