In single-ended mode you use one input only for your signal.
In pseudo differential mode, a second input is used (IN+ and IN-). On the
positive input IN+ the single-ended signal is applied.
On the negative input IN- you apply a DC voltage to shift your signal. The
voltage applied to this input provides an offset from ground or a pseudo ground
for the VIN+ input. The benefit of pseudo differential inputs is that they
separate the analog input signal ground from the ADC’s ground allowing dc
common-mode voltages to be cancelled.