BIA
BIA estimates body composition by passing a small electrical current through the body and measuring the resistance it meets.
Fat, muscle and water conduct electricity differently, so resistance can be turned into an estimate of composition using a prediction equation.
It is the method used by most consumer body-composition scales. It is cheap, quick and repeatable, and it is substantially more sensitive to hydration than DXA — enough that the same person can produce meaningfully different readings within a single day.
What it is not
BIA does not measure fat or muscle. It measures electrical resistance and then predicts composition from it. Two scales using different equations can report different results for the same body at the same moment.