xBA is the expected batting average based on exit velocity, launch angle, and (for some plays) sprint speed — strips out luck.
xSLG analogously projects expected slugging. Actual SLG > xSLG means lucky; actual < xSLG means unlucky.
Over larger samples, actual stats regress toward xBA/xSLG. Big mid-season gaps flag likely regression.
xwOBA extends the same idea to overall offensive value.