Left communism is a pseudo-Marxist tendency which Lenin famously called an "infantile disorder". It confuses what it thinks or wishes reality is for what reality actually is. They prize "principle" over reality and thus commit the inverse error to opportunists, who think any compromise ought to be allowed and ignore any principles. Both ignore the particular characteristics of every compromise, every action, etc. and reject the fundamental unity of theoria and praxis.
In "Left Wing" Communism: an Infantile Disorder, Lenin brings up the example of communists who refused to work in reactionary trade unions so that they could push the reactionary leaders out, etc., thus refusing to go to the masses where they actually were, for one example of LWC in action. These "communists" ignored the way in which the proletariat had in actual historical reality organized itself, and believed that how they thought the proletariat should have organized itself took precedent over that reality. They thus formed little "revolutionary" trade unions completely isolated from the masses and achieved no success, in contrast to the Bolsheviks who went to every single place where there were workers, even Black Hundredist meetings. Lenin put it this way:
If you want to help the “masses” and win the sympathy and support of the “masses”, you should not fear difficulties, or pinpricks, chicanery, insults and persecution from the “leaders” (who, being opportunists and social-chauvinists, are in most cases directly or indirectly connected with the bourgeoisie and the police), but must absolutely work wherever the masses are to be found.[1]
You must not sink to the level of the masses, to the level of the backward strata of the class. That is incontestable. You must tell them the bitter truth. You are in duty bound to call their bourgeois-democratic and parliamentary prejudices what they are—prejudices. But at the same time you must soberly follow the actual state of the [[class-consciousness] and preparedness of the entire class (not only of its communist vanguard), and of all the working people (not only of their advanced elements).[2]
Many modern left communists think that they are exempt from Lenin's criticism in this book since they are influenced by Bordiga and Italian left communism while Lenin in "LW"C was mostly criticizing British, Dutch and German left communists, but this is ignoring the fact that not only did Bordiga agree with the Dutch leftcoms Lenin was criticizing on some points, refusing to work in parliament, and was directly attacked by Lenin for this, but also that they still commit the same fundamental errors as these other left communists. They prize slogans like "abolish the commodity form" over material reality, and ignore how both the proletariat and AES have historically developed, judging everything by isolated (and in most cases half-understood) Marx quotes.
Communists of the Golden Center realize that principle must go hand in hand with compromise and practical action and comprehension of reality, thus avoiding both opportunism and left communism.
Left Communism is thus a strain of Leftism and is closely related to red liberalism.