Black can win a queen here for nothing, and doing it loses the game in two moves. The position comes from 1.e4 e5 2.Nf3 d6 3.Bc4 Bg4 4.Nc3 h6 5.Nxe5, where White’s knight has just walked onto e5 leaving the queen on d1 attacked by the bishop on g4.
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e h 1 This is Légal’s Mate, named after the French player who produced it in Paris in 1750. On one board you have a blunder that ends the game (5…Bxd1) and a sober move that keeps it (5…dxe5). That is the difference the three symbols are trying to catch.
The three signs
A double question mark, ??, is a blunder: a move that hands over the game, or a piece, or a mate. A single ?, a mistake, makes your position clearly worse without ending anything, like letting a knight settle on a square you can never attack again. And ?!, read aloud as dubious and printed by engine sites as an inaccuracy, is a move that is simply not the best one; you are still fine, just slightly less fine.
The marks are older than computers. Annotators were writing ! and ? in tournament books long before an engine ever graded a game, and you will still see them used by hand in chess notation, where a human decides what counts. What the sites added was a threshold: how big a drop in the evaluation turns a ?! into a ? and a ? into a ??. Every site sets those boundaries itself, so the same move can come back as a mistake on one and an inaccuracy on another.
What the label is worth
Not much on its own. Three blunders in a game is normal at 1200 and it tells you nothing about which one mattered.
The useful question is what kind of blunder it was. Hanging a piece to a one-move capture is a looking problem, and the cure is a habit: before you touch anything, check every enemy piece that is left undefended and every check they have. Walking into a two-move tactic is a seeing problem, and puzzles fix it. Missing a threat you had already spotted three moves earlier is a discipline problem, and only slowing down fixes that.
Sort your blunders into those three buckets for a week and you will find that almost all of them are one kind. Mine were the first kind for years. There is a whole post on how to stop blundering that goes through the checklist properly.