Each layer of your 3D print consists of a combination of the outer edge and the infill. The outer contour lines form a firm and solid outer edge for the print and determine the wall thickness. The infill of the print generally consists of a quick 'back-and-forth' pattern between the outer edges and fills in the rest of the layer. Due to the difference in print pattern between the outer edges and the infill, it is important that these two adhere well to each other. If you see that there are gaps between the outer edges and the infill, this can have various causes.