A person who identifies as agender is one that identifies as neither male nor female. This is in contrast to cisgender
people, who identify as the sex that is the one that they originally physically were, and transgender
people, who identify as a specific sex other than the one they physically were, among other identities.
Some agender
people have preferences for certain personal pronouns. While most
people prefer either he or she, which some agender
people prefer or don't mind, others prefer "they", "it", or a not-well-known attempt at making a new pronoun.
The argument for "they", as I understand it, is that it is sometimes already used as a singular word (such as for a person of unknown
gender and sex), but others prefer "they" remain plural. The reasoning for preferring "it" can be the person rejecting or ignoring the diminutive uses of the word, at least when it is used tastefully. Other times, both of those are found to be insufficient, so additional word choices are found or created.