Frame of Reference

$S_1$ and $S_2$ are two frames of reference moving with uniform velocity with respect to each other. A particle $P$ is observed from $S_1$ to have a velocity $V$ and acceleration $a$. How the above description of motion of the particle changes if it is observed from $S_2$? Explain your answer.

''Change of frame of reference changes the description of motions, but it does not change the occurrence of an event" - justify.

What is frame of reference? Why we do not get absolute velocity of a body?