what is relative space ?

relative space means space that is defined by relative units

relative units are units whose magnitude may vary in different locations

usually, we define space using units such as miles or meters. we assume that (at least in daily life) a meter is the same everywhere in space. in principle, instead of meters, we can also use any other kind of units or parameters.
however, many units vary in magnitude through space. they are relative units. the space defined by these relative units is called relative space.
why would one do this? it makes a lot of sense when we want to capture the subjective qualities of space. since we cannot perceive absolute distances directly, we have to use different cues to measure space.

source: ABLER et al, Spatial Organisation: The Geographer's View of the World (1973:S7)