The Arrow Case is a small cartridge that contains nine collapsable arrows that can expand and be fired at once.
When Tony Stark started to fund the Avengers, including designing and creating new technology for the team, he provided Hawkeye with an assortment of different weaponry to us in battle. One of these items was a small case with three columns, each storing three collapsable arrows.
Hawkeye wears two of these cases on each of the outside of his legs, which he is able to easily disconnect and expand all nine arrows at once. There is a grip for four fingers to hold in-between the columns, which allows Hawkeye to fire the case of expanded arrows with ease, much like his regular arrows.
During the Battle of Sokovia, when Scarlet Witch found herself having a breakdown, Hawkeye took her aside and motivated her to go back to and fight. As he headed back outside to continue fighting, he armed himself with an arrow case, loading them on his bow. He then charged outside, where he fired the arrows at the Ultron Sentries.