Because of the side-looking geometry of the SAR system, pixel resolution in azimuth and range direction is different. To get squared pixel, multi-looking is applied which averages one or more pixels in range direction with a multiple of pixels in azimuth direction based on their size ratio. Please also see here: Multilooking necessary when geocoding? - #2 by ABraun
This depends on the product level. Level 0 products are not focused, so SNAP cannot read them. SLC or GRD products (both Level 1) are already focused and can be processed in SNAP. More on this Sentinel-1 Data Products.