울다 means "to cry". It is the basic dictionary form of the word. It is the basis for all conjugations. In each conjugated form, it has a slightly different meaning. 울었어요 - cried 울고 있어요 - is crying 울어요 - cries 울고 - cry and 울지만 - cry but 울어서 - cry so 우는 걸 is contracted from 우는 것을. 을 is an object particle. It shows the function of the noun it is attached to in the sentence. 것 means "thing". ~는 것 is a grammatical structure used to turn a verb into a noun. 우는 것 means "(the act of) crying". 먹는 것 means "(the act of) eating" 골프 치는 것 means "(the act of) playing golf" 노래하는 것 means "(the act of) singing" This is the structure used when talking about your hobbies. For example, 저는 요리하는 것을 좋아해요. "I like cooking."