The Source Filmmaker (SFM) is the movie-making tool built and used by us here at Valve to make movies inside the Source game engine. Because the SFM uses the same assets as the game, anything that exists in the game can be used in the movie, and vice versa.
Most popular community and official game content for the past week.  (?)
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It means the content you're seeing has been rated up more than it has been rated down for the given time period.