This is an umbrella term for a number of Iran-backed militias, some of which, ironically, have previously fought on the same side as the US against their common enemy in the region: Islamic State. They share common aims with Iran, namely to drive the US military from Iraq and Syria and to punish the US for its military support of Israel.
Source link: https://www.bbc.co.uk/news/world-us-canada-68135774