A 0/10 represents an utter impossibility that a card will see play in a format, usually meaning the card is strictly worse than another card or so situationally worse that a hard zero quantifies its quality, like Squire in a Stoneforge Mystic world. A 5/10 represents a card with a reasonable chance of seeing play in its format in any deck that's high enough tier to be 'relevant', where relevant is defined as a sufficient meta share that a player experienced in the format will see it and won't be likely to classify it as jank. For Modern, this might mean that things that end up seeing play in, for example, Ad Nauseam are right along the relevant/irrelevant border, because a player might either classify it as jank or not, and it doesn't occupy enough meta share to be considered part of the 'obviously not jank' section like UW Control or Death's Shadow. A 10/10 is an inevitability to see play, like cycling out of existing staple lands, effects pushed enough to demand at least some use, strictly or conditionally better versions of existing cards that see play, etc.