Unaligned specifically describes the "alignment" of creatures that lack the intellectual capacity to understand morals and ethics, as the Basic Rules describes:
Most creatures that lack the capacity for rational thought do not have alignments—they are unaligned. Such a creature is incapable of making a moral or ethical choice and acts according to its bestial nature. Sharks are savage predators, for example, but they are not evil; they have no alignment.
What this is meant to convey is that a creature that lacks the capacity for moral reasoning can do things which would absolutely be considered evil (or good, or lawful or chaotic) actions, were it a creature with the capacity for moral reasoning, but that doesn't make it actually evil (or good, or chaotic or lawful).
To take an example from popular culture, the common domestic housecat has a reputation for apparent sadism and cruelty based on how it hunts and "plays with its food". Were the cat an intelligent creature who chose to act that way towards its prey, it could quite fairly be called evil; it effectively often tortures prey animals for no practical purpose. However, a cat is not capable of moral reasoning, and it is not reasonable to ascribe an alignment to it - the game makes this explicit by calling such creatures unaligned.
Being unaligned is not the same as being neutral, since being neutral describes an intelligent creature that takes an overall neutral moral/ethical stance, but an unaligned creature could display any manner of cruel, altruistic, random or ordered behaviour without that having any impact on its mechanical alignment.