Metahumans (term coined by Lex Luthor) are a class of individuals that possess extraordinary powers, traits, and abilities through either accidents, foreign exposure, or a unique genetic composition, making them considerably more powerful than regular humans.
Metahumans were under investigation by LexCorp Industries. The data that Lex Luthor collected on them was part of the LexCorp surveillance stolen by Bruce Wayne. This revealed their existence to him and then to Wonder Woman, whom he later shared the Metahuman data with.
After Superman's funeral, Batman and Wonder Woman discussed bringing together the metahumans seen in Luthor's files in the case of another global calamity. When Diana asked for the reason why Wayne wanted to assemble these people and why they would be willing to help, Bruce replied, saying that he "just had a feeling" (in reality, he was following the instructions given to him by the scarlet time-traveler that appeared in one of his "dreams").
Amanda Waller, the director of ARGUS, formed the Suicide Squad, a team of disposable incarcerated criminals with exceptional abilities, skills, and powers which could be forced to battle against formidable metahumans that threaten the human race. Though joining unwillingly, Task Force X (as the Squad was officially called), under the leadership of Rick Flag, successfully defeated the powerful Enchantress and her brother Incubus, both ancient and extremely powerful metahumans fixated on taking over the world. For their help in saving the world, the Suicide Squad members were given 10 years taken off their respective prison sentences.