Iridium is a rare element able to create anti-protons.
In order to build a portal to bring the Chitauri to Earth, Erik Selvig needed iridium to stabilize the Tesseract-created portal. So Loki and Hawkeye went to Stuttgart, Germany to steal the iridium from a secure installation.
Upon arrival, Loki attacked Heinrich Schäfer who owned a warehouse that housed the element. After Loki removed Schäfer's eye, Barton used it to access the warehouse, get the iridium, and send the element to Selvig. Selvig then used the iridium to stabilize the portal, allowing the Battle of New York to begin.