think of a series of events A, B, C
lets say event A is you sitting, event B is you standing and event C is you walking
we can describe the series of events in one of two ways; tensed and tenseless.
tensed:
at event A, event A is present, events B and C are future
at event B, event A is past, event B is present, event C is future
at event C, events A and B are past, event C is future
this is our usual way of thinking about time.
tenseless
at time t1, event A is before event B and event B is before event C
at time t2, event A is before event B and event B is before event C
at any time tn, event A is before event B and event B is before event C
tensed terms have been removed from our account of time, all that is left is tenseless terms and fixed permanent relations.
the argument goes that all time can be accounted for in this tenseless way. this means that all tensed statements can be rewritten as tenseless statements.
consider: i am hungry (am is tensed here, meaning i am presently hungry)
this can be rewritten as: at time t1 person A is hungry (is here is tenseless, like the 'is' in 'is before than')
this is the tenseless theory of time, it is designed to be compatitble with JE Mactaggart's famous 'The Unreality of Time' paper in which he claims that tensed time relies on an inconsistency and therefore time is not real.