Shimmering oil is hot oil that is nearing its smoke point.
At room temperature, common cooking oils like vegetable and olive oil seem fairly thick. Put them in a pan and heat them though, and they thin out when you swirl the pan. As they get hotter, they tend to "flow" and coat the pan more easily.
In the right light, when you look at oil that's at a good temperature for sautéing -- nice and hot, but not yet smoking -- it shimmers. It forms "tines" like those on a wine glass. It looks colorful, iridescent even.
Shimmering oil is good for sautéing because it increases the chances that the food won't stick. Hot oil immediately seals the bottom of food, creating a natural barrier between it and the bottom of the pan.