Imbrication is one of those geologic phenomenon that occur and are recorded at a scale that we can readily see. What is it? Imbrication is the orderly, overlapping arrangement of flattened or sub-spheroidal grains in the direction of flow. Flow in this case is usually water flow, but it can be other types of fluid flow [ice, wind, and even metamorphism]. Grains are commonly sand- to gravel-sized particles. However, with other types of fluid flow or metamorphism, these can often be large-scale or even microscopic.
For the purposes of this discussion, we’re going to focus on water flow and gravel-sized particles. Below is a diagram from the University of Montana Geology Department  depicting varying degrees of imbrication:
On a recent trip to Wrightsville Beach in Wilmington, North Carolina, I came across this shell bed. Can you see the imbrication in the photograph? I put my sunglasses in for scale, and some background into the scene so you could see the orientation of the beach with respect to the Atlantic Ocean: