Consensus means full agreement reached between all persons within a group or set.
The term is used by anarchists as well as many others to refer to internal decision-making processes in which full agreement is required in decisions that (significantly) affect others in the group or are made in the name of the whole group.
Generally it is understood that individuals and affinities always have the power to act autonomously, and that consensus is needed only to claim the approval of a larger body or make decisions that affect others in the body. Unfortunately, however, consensus practice often finds individuals and affinities acting subservient to the larger group and unnecessarily requesting approval for insignificant decisions or for decisions that would better be done autonomously. Individuals also often forget that if they want something to happen they may have to do it themselves. (A group cannot do anything unless individuals within it take initiative).
A common argument for why using consensus process opposes hierarchy is something resembling: "It allows us to make decisions in the process of fighting against systems of hierarchy while also not making those decisions hierarchically."
But the word "hierarchy" originally refers not to authorities having power of decision-making, but rather to spiritual beings holding a sacred power (of closeness to God). This meaning was transposed to the Catholic hierarchy (made up of persons supposedly having sacred power), and this transposition gave hierarchy an embodied and systematic force. Following this, it could be argued that hierarchy originates in the power that ideas have when held above us as sacred, and that this power can take on a social and material form. (This would be too long of a divergence to go into depth on here, but you could refer to Max Stirner for more on the topic.) Perhaps, then, the issues I described above concerning consensus, such as the inability of the individual or affinity to feel able to act autonomously from the consensus-making group, are related to a kind of hierarchy in which the sacredness of consensus can have a power over the will of each individual who's part of the whole.
Others will point to issues of social status, identity politics, and so on as "the hidden hierarchies within consensus." These claims may be true but often the approach seeks to lower everyone to a lowest common denominator called equality in which everyone is meek and inoffensive, and walking on the eggshells of their so-called privileges, which certainly is no way to live freely, much less constitute a force to destroy the immense institutional structures of hierarchy that are this society.