let’s say there is a city in your area where you have field survey data and there is no significant displacement.
Select this point in every image. Because of the relative measurement of DInSAR this point could still be -5, +15, 0 or -1 throughout the images. Now you take these values per image and add them to the result, so this point is really 0 and all other displacements in the image are levelled according to this specific points. Only then you can compare images of multiple dates.
We had some discussions on this here: Displacement miscalculation and here: Subsidence map in 3d view