The Tesseract is described as a powerful energy source of unknown potential. The film never calls it an Infinity Stone; Kevin Feige later said it holds the Space Stone.
The Space Stone goes into the Nano Gauntlet Tony builds, and Bruce uses the gauntlet to bring back everyone Thanos erased.
The glowing cube that drives the first team-up is part of the tool that undoes the Blip.
Source: Wikipedia: Infinity Stones