Maiden has two main meanings that share a common thread:

1. An old-fashioned word for a young, unmarried woman. You'll encounter this meaning in fairy tales, Shakespeare's plays, and older literature. A “fair maiden” in a medieval story is simply a young woman, often the heroine of the tale. The word sounds formal and poetic today, which is why modern writers rarely use it except in historical or fantasy settings.
2. Something happening for the first time, especially in formal contexts. A ship's maiden voyage is its very first journey across the ocean. A politician might give a maiden speech when addressing Congress for the first time. A cricket player's maiden century is the first time they score 100 runs in a match.