I ham confused on how to use qui and que since both of them can mean who, that etc

Qui is used when "who" is the subject of the clause ans que for when "who" is the direct object. Tu elucidate:

Tu connais le mec qui parle à ma sœur? -- Do you know the guy who is talking to my sister. Here the guy is doing the talking, so qui, referring to the guy, is the subject. 

Tu connais le mec, que ma sœur a rencontré hier? -- Do you know the guy who(m) my sister met yesterday? Here the guy is being bet by my sister. Hence my sister is the subject and the guy the direct object to which que refers. 

-- Chris (not a native speaker).