Stockfish and other mainstream engines will judge a position in units of centipawns - one hundredth of a pawn. Largely this will be to do with material on either side. For example, if you throw away a pawn with all else being equal, I'd be up 100 centipawns. If I'm white this gets written as "+1.00", and if I'm black it's "-1.00". The score is also based on a strategic evaluation of the position (and positions to come) using (in Stockfish's case) some heuristics. It's worth noting that it's this evaluation of a position at which AlphaZero appears to be three orders of magnitude better than Stockfish.
In some of the games vs AlphaZero, Stockfish makes errors that it _itself_ appears to be able to judge as huge blunders. In game 3 (which people view as a masterpiece of long term strategic thinking by AlphaZero) one of these is at least a +10 swing to AlphaZero as white. That's about the same as throwing away your queen. Without the weird time controls put in place, it seems unlikely that we'd be seeing blunders like that. As I said before, I'd still expect AlphaZero to win, and in many cases it was already ahead before these mistakes, but it's worth mentioning in any analysis.
In some of the games vs AlphaZero, Stockfish makes errors that it _itself_ appears to be able to judge as huge blunders. In game 3 (which people view as a masterpiece of long term strategic thinking by AlphaZero) one of these is at least a +10 swing to AlphaZero as white. That's about the same as throwing away your queen. Without the weird time controls put in place, it seems unlikely that we'd be seeing blunders like that. As I said before, I'd still expect AlphaZero to win, and in many cases it was already ahead before these mistakes, but it's worth mentioning in any analysis.