It's pretty clear that Marvel didn't really have a tight grip on all of the finer details of the Infinity Stones back in the early days of the MCU, but some of their missteps stick out more than others, like the way the Tesseract is explored and explained in The Avengers.
In the opening scene, we're introduced once more to the Tesseract, which SHIELD has been examining to the extent that Hawkeye is somehow an expert in it. It is he - and confusingly not Erik Selvig - who offers an explanation on the Tesseract being a doorway capable of intergalactic wormhole travel. He also talks about the Tesseract needing something else on the other side to make it work - mostly as a means to explain how Loki uses the Tesseract to get to Earth without having it in his possession.
So, what exactly is it that works with the Tesseract to help Loki get to Earth? What is the other side of the door? Unfortunately, after it's mentioned (to assist the plot), it's then just ignored entirely.