Have you seen this topic? Terrain flattering, Sigma0, Beta0 and Gamma0
There is also an explanation here: Radiometric & Geometric Correction Workflow
And one by @peter.meadows: Nominal incidence uncorrected sigma nought
Lastly, there is the radar and SAR glossary by ESA: https://earth.esa.int/handbooks/asar/CNTR5-2.html
Please go through these and feel free to continue here if anything remains unclear.