Light cone

future and past are well defined: every reference frame sees your future in your future cone, and your past in your past cone

Otherwise causality could be violated, and then things would go really bad, you could tell your past self to tell your past self to tell your past self to do something.

You can only affect the outcome of events in your future cone, and you can only be affected by events in your past cone. You can't travel fast enough to affect.

Two spacetime events with such fixed causality are called timelike-separated events.

every other event (to right and left, known as spacelike-separated events) can be measured to happen before or after your current spacetime event by different observers.

But that does not violate causality, because you just can't reach those spacetime points anyways to affect them.