Ultra is short for Ultra Left, which very broadly speaking can be considered a form of what Lenin called “Left-Wing” Communism.

There are a lot of different kinds of ultras, and depending on who you ask you will get a different answer as to who the label applies to (Anarchists, Trotskyists, Maoists, etc.), but what generally unites them all is their idealism, their dogmatism, and their focus on ideological purity. They reject materialist analysis that doesn’t conform with their ideology and demand the skipping of stages in the development of socialism. Ultra-leftism is often associated with adventurism and hyper-militancy.

Ultra-leftism in many ways is simply the opposite side of the same coin to which right-opportunism belongs. They are undialectical deviations from Marxism. Because of their shared lack of solid dialectical grounding, sometimes former ultra-leftists, feeling scorned or frustrated, can flip straight into becoming right deviationists (capitalist roaders), or worse. Their danger stems from the fact that both can steer revolutionary forces and socialist governments off-course and into dead ends, though they do so in different ways.

“It should not be forgotten that Rights and “ultra-Lefts” are actually twins, that consequently both take an opportunist stand, the difference between them being that whereas the Rights do not always conceal their opportunism, the Lefts invariably camouflage their opportunism with “revolutionary” phrases.”

The Fight Against Right and “Ultra-Left” Deviations, - J. V. Stalin

Ultras are the kind of people who would say that China is bad not because it is communist but because it isn’t communist enough. They want full blown communism to happen right now, which is understandable as a desire, but then they don’t want to engage with a process of transition, based in dialectical materialist theory and practice, to get there. Those who actively engage in trying to transition to communism invariably find they can’t change everything and everyone all at once, and on top of that, are surrounded by violent capitalists/imperialists who want them dead. So they have to contend with that and work out an effective strategy to move toward communism anyway. This invariably involves some compromising and negotiating along the way. Not to be confused with the kind of compromising that liberals talk of in “progressive” politics. For members of a vanguard party belonging to a socialist state, compromising is not bowing to capital or empire, but figuring out how to deal with the fact that those forces exist and develop socialism and then communism anyway. For "progressive’ liberals, compromising means giving up on anything revolutionary and satisfying yourself with minor reforms within the oppressive status quo.

I note the distinction in part because some ultras would appear to think these are the same thing, that the compromises a socialist state makes are the same type of concessions that liberals make. But the fundamental difference is in where the power lies and who controls the means of production and distribution. That’s what you want to be looking for, primarily, not whether every policy of a socialist state exactly lines up with communist ideology at the state’s current stage of development.

They have a tendency to show up in the western left because the west has yet to actually do a socialist state project and because decades of vilifying socialist states has put a stigma on supporting them in any way, but leaves it as moderately acceptable to support communism in the abstract, as long as communism only exists as a nebulous idea that “hasn’t truly been tried yet.” In other words, believing in an idea is something the western empire is more or less okay with, since action doesn’t necessarily follow from it. But if you start supporting the entities that are standing up to it, whether these entities are explicitly communist in intent or are just trying to be sovereign and self-determining, it gets mad fast.

The TL;DR is basically: Ultras, or ultra-leftists, are self-proclaimed communists who are extremely dogmatic about doing everything exactly as it was written to the letter even if it’s not possible in the given material conditions, while also ironically often not having read much theory and/or not understanding theory very well.

Here’s what prolewiki has to say about ultras

what ExotiqueMatter said with the addition of in some circles it is used incorrectly as a thought terminator the same way “tankie” is among anarchists/liberals
that’s mostly among baby MLs though