First for all, you should not using "the" before T.V. It means you are looking at the machine, whether it is turned on or not. The first one means that you were talking about the past as if you are still doing it as if in the present. The second one means that you were describing a finished past action.

I'm not sure, but let's try: the first one means that I stopped watching TV when my friend arrived. the second means that I am still watching TV even when my friend arrived.

1) I was watching the t.v before my friend came. 2) had been watching the t.v before my friend came. Let's isolate the only variable between these two sentences: 1) past progressive tense 2) past perfect progressive tense The only difference is the speaker / writer's use of the perfect aspect. The perfect aspect shows something (arguably) SUBJECTIVE i.e. the speaker / writer's perception of a past action as somehow still relevant to the present moment. 1) speaker / writer is just indicating they see the 'watching' as having taken place over a period of time e.g. a few hours -- as opposed to saying "I watched TV", (simple past, just indicates a completed past action) 2) speaker / writer adds the perfect aspect, meaning that he or she wants you know that 'watching' is connected to whatever the next action is / whatever is going on at the moment of speaking / writing The context would be something like: I had been watching TV for several hours before I realized it was Monday and I should be at work. (If the speaker / writer *doesn't* add the perfect aspect here i.e. if she just wrote 'was watching' ... the connection between the two actions in the sentences could be ambiguous / other possibilities aren't definitely rejected) There are SIX contexts where this tense is used but that's too much for here. One example: (to describe new and developing situations) I've been putting on weight. I've been thinking about a new job. Textbooks often use the word 'continuing' when talking about the perfect tenses, which messes people up because the perfect aspect is not that the ACTION itself 'continues' but that the relevance / the impact of that action 'continues' from when it occurred, to the present moment. The use of the word 'continuing' to talk about progressive / continuous tenses is incomplete / misleading / a cause of confusion. There are great ENGVID / Jennifer ESL videos on this but I just wanted to answer this question because it comes up all the time.