When you add a raster dataset that does not have pyramids or statistics into , you are prompted to build pyramids and calculate statistics.
Build pyramids
Pyramids are reduced-resolution overviews of the data at different scales. Pyramids are useful because they improve the drawing speed for raster datasets displayed at less than their full resolution. It is recommended that you always build pyramids for large raster datasets.
Note:
Raster datasets smaller than 1024 pixels in both the x- and y-dimensions will not have pyramids generated automatically; however, you can manually specify a pyramid level to override this behavior.