The circled part of the image is assorted bedrock, called sedimentary conglomerate in the geological world. The area, named 'Hottah' for Hottah Lake in the Northwest Territories in Canada. Hottah has pieces of gravel that are shaped in a formation that is round, indicating kind of a sanding-down of the material. Similar to when you see a rock that is round and smooth, these pieces of gravel are also smooth. This smoothless and round shape indicates that a water stream did affect these pieces of gravel.
This is a crucial piece of evidence, that may very well lead to another significant discovery on Mars.