The products are either available as Single Look Complex (SLC) or Ground Range Detected (GRD) processing levels.
The main difference between the two is that for GRD products, a processing of multi-looking is applied.
This process is an averaging of “sub looks” computed from the full resolution SLC data. Each sub look is “detected” before the averaging (thus the D in GRD).
The Equivalent Number of Looks (ENL) is intended to correspond to the number of equally-weighted, statistically independent looks which would produce the same speckle statistics as the processing used to generate the product.
The ENL will normally be less than the total number of looks, NL, because of partial overlapping of the looks and unequal weighting.
You can here refer to: https://sentinel.esa.int/documents/247904/1877131/Sentinel-1-Product-Definition
Using multi looking allows to improve the speckle statistics, but reducing the resolution.