Lo is an complement of Direct object, it is used in English as an object pronoun, the difference is that in english you say, "lo que": "what"; "lo uso": I use IT. In Spanish this kind of Object Pronoun is, most of the times, used between the subject and the verb.

Here's your answer: http://spanish.about.com/od/sentencestructure/a/lo.htm

Example: I did it : Lo dije "Lo" is the same that "it" at the end of the sentence.