A direct effect measures how much the dependent (outcome) variable changes when the independent (predictor) variable increases by one unit.
Pearl, J. (2022). Direct and indirect effects. In H. Geffner, R. Dechter & J. Halpern (Eds.), Probabilistic and Causal Inference: The Works of Judea Pearl (pp. 373-392). ACM Books. https://doi.org/10.1145/3501714.3501736