They mean that the length of a meter stick, or the rate of ticking of a clock depends on who measures it.
(They; mean; that the length of a meter stick, or the rate of ticking of a clock depends on who measures it) Context(They mean,List([0, 9))):(the length of a meter stick, or the rate of ticking of a clock; depends; on who measures it)
They mean,List([0, 9))
They the length of a meter stick, or the rate of ticking of a clock
mean depends
that the length of a meter stick, or the rate of ticking of a clock depends on who measures it
on who measures it