An Endcap is a product/PoSM placement shelf at the end of an aisle in a retail outlet. It is believed that products placed at endcaps attract more customers relative to others placed somewhere on the regular shelves. Retailers tend to rent out the endcap space to brands which benefits brands in improved sales. Brands tend to use End Caps to highlight their preferred products or new launches. Since a large number of customers will walk by an Endcap, they are also good areas to display products for impulse sales.
Following is a picture illustrating an end cap: