A cataract is an area of a river where the elevation of the streambed changes too quickly or has to many boulders meaning people couldn't use part of the river as transportation.
This was a good thing because the cataracts effectively served as barriers so they could control travel. This meant that the Pharos found it impossible to control the land south of the first cataract. This ending up becoming the southern border of Ancient Egypt.
Hope this helped ☺️
