Performative activism is a pejorative term referring to activism seemingly motivated to "perform" (only for shows, not genuine), to increase one's social capital, rather than motivated by one's devotion to a cause.
Examples of what people have deemed performative include posting black squares in social media during Black Lives Matter, the "tax the rich" dress worn by left-wing politician Alexandria Ocasio Cortez at the Met Gala, etc.
A leftist is someone from the general community, maybe a student, an activist, a commoner, or a leader, who is supportive of left-leaning economic and political principles such as low government interference in economic life, wage equality, strong labor unions, and so on.