Body effect refers to the change in the threshold voltage of the device when there is a difference between substrate(body) and source voltages. Body bias is usually the lowest voltage in the chip(in case of of p-substrate).
Let’s look at the NMOS given below.
However, if we were to connect Vbult to a voltage lower than VSS (Source voltage), there is an increased flow of carriers between these source-bulk junstion thereby increasing the width of the depletion region. This in turns increases the minimum gate voltage needed to achieve channel inversion.