Mass/energy as a "condition of spacetime"
I recently read somewhere (I think here) that mass is not something you "drop into" space, rather it's a condition of space, and that gravity waves are also a "condition of space".
a) is this correct?
b) If so, is it more generally correct to say that mass/energy is a "condition of spacetime", and specifically that the invariant-mass component is a condition of space, while the energy component† is a condition of time?
c) if question b) is in fact meaningless, is there a way to rephrase the statement "mass is a condition of space" so that it that it involves time as well as space?
†I forget whatever you call relativistic mass minus invariant mass because I'm Alzhammered.