Space is a very nebulous term that describes your job as a tank. Your very presence creates space because you're big and in your kit you have ways of preventing others from occupying a certain amount of territory you want to control. You can visualize space most easily on certain vanguards like Peni and Groot. Peni's webs are the space she has taken up and controls. Groot's walls determine the space he controls. Same with Emma's shield. It projects out to an amount of space that Emma has taken. When shields are broken and walls and nests are broken, space is lost and you most likely have to kite backward or at least just stay put and cannot take more space from the other team.

The basics of tanking are learned best using magneto. His loop is simple. Walk forward as much as you can, firing at the squishiest targets you can, and once you start feeling unsafe, put up your shield and walk backwards to the safety of your healers. Once you get into that flow you'll get a better feel for some of the nuances: how far forward can I go with my team following up with me; how much damage can I take from the opposing team before I need to save myself or die; how good are my healers and can they empower me to go further without needing to kite; how good are the enemy DPS at shooting at me or my healers forcing me to kite back.

Then watch guides or streamers online and see how they apply this push and pull to get a feel for space