They are people who have left-wing views on policy, diversity, and economics, but who see "the left" as flawed and do not wish to be associated with them. Common complaints include that "the left" is too invested in organisation, identity politics, and morality. Max Stirner, the Egoism guy, is a central figure in post-leftism.
Critics of post-leftists say that they're just leftists who want to seem different and hip and cool. The fundamental post-leftist assertion is that "the left" is a monolithic group of old fashioned people who never accomplished much good, and I disagree strongly. To Me, leftism cannot ever describe a single group, but instead describes a relation between a given group and a reference point; the left is a direction, not a place.