What is a difference between newton’s laws of motion and einstein’s theory of relativity?

Strictly speaking, neither theory explains the Universe. Each theory provides a mathematical model, and the model predicts (and perhaps explains) the behaviour of stuff in the Universe. In Newton’s theory, time and space are absolute, separate from each other, and given, which is exactly as they appear to us humans; and they are not explained in any way. They form a geometrical structure through which matter moves. In Einstein’s theory of Special Relativity, time and space are tangled together. As one example, two events seen by one observer as simultaneous. When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.