If you go online and search, "I need some space," you'll see some truly bleak relationship diagnoses. What does it really mean?
Honestly, I think it depends on the person saying it. If you've been casually dating for two weeks and your Other pulls out the space card, perhaps you shouldn't get too invested. If you've been married for almost three years and together for a year prior to that...well, clearly I'm not enough of a relationship expert to know whether or not that means your Other wants to break up, has some new piece taking your place, but doesn't want to say the words and hurt you, and so is trying to create distance in the relationship to make the break cleaner, or if your Other needs to think about things and do some self-analysis.
What I can tell you is that as a female who has tried very hard to avoid emotional pain in life, to the point of refusing to date only one person at a time until Mr. Right touched me and made me feel the feeling I thought was a bunch of hokey you only get in romance novels, the whole space thing may be a deal breaker.
When a relationship is in danger of ending, and Other wants space, all I see is Other ending things, and the breakup grief begins. There's self-blame with bargaining and sadness, a whole bunch of anger, and if the "space" of Other pushing me away goes on too long, eventually, I just become numb to the whole thing. The opposite of love isn't hate, because that is an equally strong emotion. The opposite of love is indifference, and the scary part of "I want some space," is wondering whether or not I'll be able to get through the grieving with enough love intact if Other does decide to try to make things work.