It's a term for those who make dogmatic or adventurist errors in judgement. In practical example, there was the famous theory of "social imperialism" as supposedly practiced by the USSR. This theory did not correctly grasp the nature of imperialism nor the Soviet Union's actual material processes, but served as a wedge by which those could oppose the Soviet Union while adhering to nominally left politics. Essentially, ultraleftism is confusing allies for enemies, which very frequently turns into its opposite, confusing enemies for allies, like those on the "left" that support NATO against the PRC because the PRC has supposedly "taken the capitalist road."