As glaciers flow over land, they incorporate pieces of rock and sediment into the ice. These inclusions make the glacier sole (the bottom of the glacier) into a kind of coarse sandpaper that is capable of scratching bedrock. Over time, the glacier moves over rock and sediment, leaving striations or striae, on the rock surfaces that can reveal the direction that the glacier was flowing.
Glacier scientists often use striations to determine the direction that the glacier was flowing, and in places where the glacier flowed in different directions over time, they can tease out this complex flow history by looking at the layered striations. Thus, glacier striae are an important tool for people looking to reconstruct past glaciers.
As the glacier passes over a surface over longer and longer periods of time, it smoothes it much like sandpaper does. This smooth rock surface often has striations imprinted on it as well, and the entire thing is known as "glacier polish."