ascending and descending are simply the direction of movement of the sensor. Acending is from southpole to northpole and descending vice versa. As Sentinel-1 is right-looking this simply determines from which side the image is acquired. This makes a difference if you have strong topography in your image with foresthortening, layover and shadow effects.
source: https://site.tre-altamira.com/insar/
About the preprocessing: Please have a look in this topic and describe your problem there (what data you are using, what you applied and what error message you get): Radiometric & Geometric Correction Workflow