Dajun 大君: ‘great ruler’, respectful title for the king:
大君若不棄書之力… (…) 惟大君命焉.
O great ruler, if you have not forgotten the zealous duty of Shu… (…) It is for you, O great ruler, to issue your command.”
(Zuo Zhuan 左傳, tr. James Legge, p. 491)
Tang, as well as Tai Jia, Zu Yi and Wu Ding were the great rulers below heaven.
Compare with a similar sentence in Yanzi Chunqiu 晏子春秋:
Now Tang, Tai Jia, Wu Ding and Zu Yi were the grand rulers below heaven.
You ming 有命: receive an order or appointment, often as a reference to tianming 天命, a mandate of Heaven to the (self-titled) ruler, but also used in a more general sense, ‘to receive an order (from the king)’.
Kaiguo 開國: establish a feudal state (建立諸侯國). The Shanghai Museum manuscript and the Mawangdui manuscript have qi 啓 for kai 開. The Shanghai Museum manuscript has bang 邦 for guo 國:
For qi bang 啟邦, “to open the country,” R (=the received text HM) reads kai guo 開國, kai 開 replacing qi 啟 to avoid a Han-dynasty taboo on the name of Liu Qi 劉啟, Emperor Jing 景 (r. 156-141 B.C.), and guo 國 replacing bang 邦 to avoid a taboo on the name of Liu Bang 劉邦, Emperor Gaozu 高祖 (r. 202-195 B.C.). M (= Mawangdui manuscript HM) reads qi guo 啟國, observing the taboo on the name of Liu Bang, but not on that of Liu Qi, while F (= Fuyang manuscript) reads as does the Shanghai Museum manuscript.
– Edward L. Shaughnessy, Unearthing the Changes, p. 78
The phrase qi bang 啟邦 is also found in a bronze inscription, where it is read as ‘to expand the country’ (金文常用字典, p. 361; see image in 殷周金文集成 15.9734).
Cheng 承: continue a heritage (秉承). The Shanghai Museum Manuscript has cheng 丞, a common loan for 承.
Jia 家: ‘family’, but might also refer to the nation or country.
Xiao ren 小人: people of lower standard – commoners, those that are ruled instead of the rulers, those with narrow minds & views.
The great ruler received the order to expand the country and continue the nation he inherited. People of lower standard should not be used (for this).