Pan-leftism is an ideology, or the lack there of one, which promotes the notion that Anti-Capitalist Leftists of all shades should cooperate instead of constantly bickering amongst themselves.
Pan-leftists welcome all Anti-Capitalists be they Anarchist; Communists of all shades including Maoists, Trotskyists; Religious Socialists (Such as Liberation Theologists or Islamic Socialists); Democratic Socialists (of the sort who don't support the Capitalist means of production); Syndicalists or any other ideology which supports the working class against Capitalism and Fascism.
Marx is famously quotes as saying "workers of the world, unite!"