Thanos' Armor is a battle suit that was used by Thanos while invading planets in his quest to kill half of the universe.
After completing his grand goal, Thanos began utilizing his armor in a more peaceful manner, using it as a makeshift scarecrow near his house on the Garden. After the surviving Avengers ambushed and killed Thanos, they presumably left the armor behind.
When Nebula and War Machine traveled back in time to 2014, a younger Thanos, still utilizing his armor, caught wind of their plot to gather the Infinity Stones in the past to undo the Snap and used their own technology to travel to 2023 to stop them.
Thanos continued to wear his armor during the ensuing battle with the Avengers, but its helmet was destroyed when Captain America had struck him with Mjølnir, and it later sustained heavy damage when Scarlet Witch attempted to crush it along with its wearer using her powers. In the end, the armor was disintegrated along with Thanos when Iron Man used the Infinity Stones to destroy the alien army.
Thanos' Armor was created from a strong metal. This makes it durable enough to withstand the extremely powerful blows of the Hulk. However, it does have its limits, as Scarlet Witch was able to badly damage it using her powers and it was destroyed along with its wearer by the Infinity Stones.