The Huns were nomads who lived in Central Asia, Caucasus and Eastern Europe, they contributed to the fall of the Western Roman Empire because they started the Mass Migration.
The mass migration was a main factor on the Western Roman Empire because it weakened the Roman Empire's cultural, economic and military development, leaving them exposed for other countries to attack.
The Hun invasion caused a massive migration, they moved The Goths from their homes causing the Goth invasion of the Roman Empire. This is known as the Domino Effect in the fall of the Western Roman Empire.