China has become one of the most important players in the global economy. "不可或缺"means "indispensable", but this is not right in this context. I have picked "one of the most important" because that is how an educated native writer would write to convey the sense of a major player. We could use "ubiquitous", but that only means China is "everywhere", meaning "having her fingers in many pies". To be involved in many things does not always mean you are important. So I have decided to call a spade a spade.

like "side" or "party"

端 是个 十分 文雅 的 词。 with 许多 意思和意味。。。。。。 here it is a starting or ending piont or side or part. also。 meaning the or cause of something orsome phenomenon。 beside， what popularly used in thatway feng said. when you say 端上来，meaning holding up to give to you...or to put a dish on a table. here the 端 is used as verb. also with some sort of two sides' meaning.