The Outriders are supporting antagonists of the 2018 Marvel film Avengers: Infinity War. They were a mindless species of aliens subservient to the infamous Thanos, the Black Order and their sister species the Chitauri.
In his quest to collect the six Infinity Stones, Thanos collected the Power Stone (the Orb) by destroying the planet Xandar, and the Space Stone (the Tesseract) by slaughtering all of the Asgardians (including Loki). After collecting the Reality Stone (the Aether) from the Collector and the Soul Stone from the Red Skull, Thanos sends the Black Order to Earth to collect the Time Stone (the Eye of Agamotto) and the Mind Stone.
Though Ebony Maw captures Doctor Strange (the current holder of the Eye) and delivers him to the planet Titan for Thanos, Corvus Glaive and Proxima Midnight failed to steal the Mind Stone from its current holder (the superhero Vision). Even when Maw ends up being killed by an arriving Iron Man, Thanos collects the Time Stone after defeating several heroes. With victory at hand, he orders Glaive, Proxima and Cull Obsidian to lead an Outrider attack on the country Wakanda, knowing that Vision is currently residing there trying to remove the Mind Stone and destroy it.
To that end, the Black Order summons several giant ships containing countless Outriders to the scene, even when the Wakandan warriors and several Avengers activated their vibranium shields to hold them, several Outriders were able to go through shields without even dying. This forces the Wakandan warriors and the Avengers to fight back against the Black Order and the Outriders in order to buy enough time for Vision to remove the Mind Stone.
At first, the heroes are being overwhelmed as they are too many Outriders to fight back. Fortunately, Thor arrives to the rescue, bringing in two Guardians of the Galaxy (Rocket Raccoon and Groot) and a new powerful weapon called Stormbreaker, allowing the heroes to finally defeat the Outriders and kill them all as well as the Black Order, allowing enough time for the Scarlet Witch to destroy the Mind Stone.
Unfortunately, despite the loss of the Black Order and the Outriders, an arriving Thanos foils this by defeating the Avengers and Guardians and using the Eye of Agamotto to reverse the event, allowing himself to collect the Mind Stone while killing Vision at the same time. Even when Thor managed to use Stormbreaker to violently stab Thanos in his chest, Thanos used the power of the Stones to wipe out half of the universe's populace (including several Wakandan warriors, Avengers, and Guardians) in retaliation, taunting Thor that he should have aimed for the head instead. As a result, the surviving heroes mope over their failure while a triumphant Thanos escapes and oversees a sunrise from another planet.