Direct action refers to taking a quick action against something disliked. It thus excludes only expressing views against what is disliked and it excludes actions taking a long time to start and/or complete. The "direct actions" are often quasi-legal or illegal, such as sit-ins, squatting, blockades, "civil resistance", intimidation, sabotage, property destruction, and/or violence.
Although the phrase has been applied to various actions by a wide variety of groups, in practice the phrase is mainly used by far left groups and individuals, especially social anarchist ones, as a euphemism for their quasi-legal and illegal activities. Several antifa groups even name themselves "Anti-Fascist Action", or something similar, with "Action" referring to "direct action".