This is a tool-assisted speedrun. For more information, see http://tasvideos.org/957M.html
Duke Nukem 64 (1997) is a classic PC shooter ported to Nintendo 64 — unfortunately, in a heavily censored state, and without much concern for the game's performance.
As such, the movie is played on the easiest level in an attempt to reduce the framerate drop issues arising from big amounts of enemies shooting at Duke. However, the sheer speed the player is blasting through levels will likely confuse you regardless — we recommend reading the author's comments (http://tasvideos.org/1675S.html) to understand all the tricks he used.