left-communists, who are against concessions of any kind, usually with a very idealist vision of socialism. lenin wrote a pamphlet about it "left-wing communism: an infantile disorder"

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."

Ultra-Leftists are typically other communists who are not rooting their actions and methods in material reality. This false consciousness held by ultra-leftists leads to incorrect theoretical and practical conclusions. Conclusions that can endanger the movement itself. Practical concessions need to be made if the reality of the situation demands it. Ultra-Leftism, an Infintile Disorder by Lenin, covers this topic in around 90 pages. Worth reading if you haven't. Marxism is a living science and, as such, can adapt and evolve as conditions change. To not do so would lead to dogmatism and inflexibility, which undermine the communist movement.

It is the kind of people who will shoot Lenin for Brest-Litovsk treaty.

Usually other communists