just to help limit the scope of responses that are feasible, in the HSC Physics syallbus, we concentrate on the Special Theory of Relativity, which involves inertial frames of reference, the constant speed of light, and their implications (e.g. theory of simultaenity, twins paradox, dilations, contractions, etc...).
We do not ever talk about the General Theory of Relativity, which is an explanation of how the universe works in terms of gravity.