Inaccuracies are the mildest form of chess error, occupying the space between a mistake and a perfectly played move. Engine analysis might evaluate an inaccuracy as costing 0.3 to 0.5 pawns of evaluation, compared to a mistake (0.5 to 1.5 pawns) or a blunder (more than 1.5 pawns). These thresholds vary by analysis tool.
At the amateur level, games are filled with inaccuracies because players lack the calculation depth and positional understanding to find the best move consistently. Even at the grandmaster level, inaccuracies are common because chess is simply too complex for perfect play by humans.
While individual inaccuracies have small effects, their cumulative impact can be significant. A series of inaccuracies can gradually shift the position from equal to slightly worse to worse to lost. Improving the quality of routine decisions — reducing the frequency of inaccuracies — is a hallmark of progressing from intermediate to advanced play.