Final answer:
The 'Sierra' maritime signal flag means that a vessel is moving astern, or moving backwards, an important signal for safe navigation to avoid collisions.
Step-by-step explanation:
In maritime signal flags, the flag "Sierra" stands for the message: "I am moving astern; moving backwards." This is a crucial communication for maritime navigation to indicate to other ships that a vessel is reversing its course. This communication is essential for preventing collisions and ensuring safe passage in busy shipping lanes or congested harbors. Signal flags are part of a system known as the International Code of Signals (ICS) used by vessels to communicate important messages regarding their intentions and condition.