Jin's got the answer for you. There are many words in Korean that base its form in chinese character. Koreans has two way of reading chinese characters; by meaning, and by sound. For example, 老(늙을 노) means 'old'(늙다) and is read 로or노. Like wise, 者(놈 자) means 'a person'(놈) and is read 자. It's easy to understand relationship between chineses character and Korean if you kindda think it like what Latin is to English. Like, aqua means water, but is read aqua, right? So like that, 者 means 'a person'(놈) but is read 자. -But the meaning of 놈 has changed as time went and now it bears some disapproving meaning. (Note that although in Korean language word 놈 is somewhat tainted, the chinese character 者 just means 'a person,' even though we call the meaning of the character 놈.) 원래 simply means originally. If I translate the sentence it'd go like this: 노약자, 자 is 놈. It's originally 놈 자(者), and it means 'a person.' What the writer is trying to say is that 자 in 노약자 is 者(놈 자.) That is, the 자 in the word actually means 놈.

This sentence is explanation about "老弱者". Korea used Chinese words and each one has meaning and sound. 놈is 者's meaning and 자 is sound. 원래 means originally. Long time ago, people were called 놈 as a person(사람) . Nowadays, we don't say 놈. It's almost bad word... call someone like a slavery. So 者 means just a person.