These notations are meant to identify the original manuscript source of Bible passages. Here are more details from the New King James Version preface:
Where significant variations occur in the New Testament Greek manuscripts, textual notes are classified as follows:
These variations from the traditional text generally represent the Alexandrian or Egyptian type of text [the oldest, but sometimes questioned text]. They are found in the Critical Text published in the Twenty-sixth edition of the Nestle-Aland Greek New Testament (N) and in the United Bible Society's third edition (U), hence the acronym "NU-text."
This symbol indicates points of variation in the Majority Text from the traditional text [a consensus of most Greek manuscripts]. It should be noted that M stands for whatever reading is printed in the published Greek New Testament According to the Majority Text, whether supported by overwhelming, strong, or only a divided majority textual tradition.