Even we ourselves can not explain its meaning in this one word, it has meaning when it compounds to a 词 ,like 联系means connect, 系列 means series,系鞋带 means tie one's shoes,,,,,,,,if somebody quickly speak a word with pronunciation of xi, I would be confused and asked him which xi he is talking about. Sometimes one word has no exact meaning, only to become 词 it represent something.

系 as 'to be' is a very formal and bookish expression in Mandarin and ,as bear234 said, seen in titles of news only now. (But it is very common in Cantonese as I know.If you use 系 as 'to be' in daily talk, you would sounds a bit like a Cantonese speaker). 系 have many other meanings in many other words:系鞋带、中文系、关系、联系,etc.These words are much more common.

系 means 是 but it is ONLY used in titles of news.

u d better not to use this word, it always easily make mistake

in this sentence it means 是