The "Plain Red Banner" was one of the Qing's Eight Banners.
English: "Plain Red Banner"
Mandarin Chinese: Zhènghóngqí (正紅旗)
Manchu: gulu fulgiyan gūsa
The Eight Banners (Bāqí (八旗) in Chinese or jakūn gūsa in Manchu) were administrative divisions under which all Manchu households were placed. Manchus were typically born under a certain banner and served under that banner for life. In rare cases, families were moved from one banner to another. There were also Eight Mongolian and Chinese Banners.
"Bannermen" enjoyed privileges like steady payment in silver, stipends of rice and land grants, and exemption from torture when caught for a crime. In return, Bannermen were the emperor's servants and could only become warrior or official. Every banner family was to provide a number of warriors and take care of a certain number of horses.
Bannermen lived in Beijing's inner city surrounding the imperial palace, or in one of the many walled garrisons throughout the empire. The Eight Banners served as an elite front-line army in the many Qing wars of conquest and expansion.
The full article on Bāqí (八旗), "the Eight Banners".