what is dx/dt?What does it mean ? and what is limit?

dx is a small change in x, with the change approaching zero.
same with dt

dx/dt is the instantaneous rate of change of x with respect to t. You can look at it as the slope of the x versus t curve at the point t.

 

The concept of a "limit" is used to describe the value that a function or sequence "approaches" as the input or index approaches some value. Limits are essential to calculus (and mathematical analysis in general) and are used to define continuity, derivatives, and integrals.