Sahidic is the Upper Egyptian dialect that served as the literary standard of Coptic from roughly the third century to about the tenth, and it is the dialect in which most of what matters for the study of early Egyptian Christianity was written. The earliest Coptic biblical translations are Sahidic, and they are early enough to bear on the text of the New Testament, because they were made from Greek manuscripts older than most that survive in Greek.
Bentley Layton's grammar is the standard modern description. Coptic is the last stage of the Egyptian language, written in the Greek alphabet with a handful of signs taken from Demotic for sounds Greek lacked, which is why the language of the pharaohs can be read aloud by anyone who knows Greek letters. The vocabulary is heavily Greek as well, so a Coptic sentence often carries an Egyptian grammar with Greek words for the theological terms sitting inside it, which is itself a record of how the religion arrived.
The monastic literature is Sahidic too. The rules and letters of Pachomius, who organised communal monasticism in the fourth century, the sermons of Shenoute of Atripe, who is the greatest prose writer the language produced and who wrote at enormous length, and the sayings of the desert fathers in their Coptic forms all belong to this dialect. Monasticism was invented in Egypt, and a large part of its founding documentation is in Sahidic.