What it means is, something which is encompassing ‘everything’ (infinite) and we take ‘everything’ (infinite) out from it, it will still remain encompassing ‘everything’ (infinite); i.e. ∞ – ∞ = ∞

Fun aside, If you look closely, what this shloka is trying to say is, this whole universe is made up of only one thing, one energy, which some call god, divine spirit, natural force etc. This energy fills everything (pruna) in the universe, including all living and non-living things. Even though we humans, try to create distinctions like this-is-mine-that-is-yours, in reality everything is still one. Like calling water and mist, doesn’t make them different, as they are still water.

If we take that as idea one level further, all the fights we do with each other, all the hate we have will seem futile, and world will be a peaceful place, as there is no longer you, me or someone else, it’s only me/self everywhere in everything. This is the goal of vedanta to enable everyone to see themself in everything and everything in themselves, including that divine spirit or god, whatever we call. Anyone who gets to that level will be in peace, what ever happens around them.

This shanti mantra (peace prayer) from Isha Upanishad, is the epitome of advaita vedanta. BTW, When I read this shloka first time, several years ago, I couldn’t sleep for days, and was laughing all time on foolishness I was living in; and btw, that was not the last time.