It's to say that the album has two very separate halfs. One is very early 70's sounding, with the heavy-rocker 'One of this Days' and the soft 'A Pillow of Winds' as highlights, really a very experimental and unique side, if not reaching the standards for a Pink Floyd's album side, and the other half.
This one contains an only track, 'Echoes' which is for me the biggest achievement from Pink Floyd as a track, maybe along with 'Shine on You Crazy Diamond'. Not only that, but maybe it's one of the long tracks that will have to be exhibited in the future to explain people what "was" symphonic rock. The theme starts with a submarine-radar sound lost in the immensity of the ocean's silence. Some sonsensy keyboards start to groove and a beautiful guitar from mister Gilmour draws a slow and sinuous melody, and the band starts playing over Nick Mason's lazy drum beating. So the vocal part appears with a nice and sensitive harmony between Gilmour and Wright. Everything develops to an almost pre-funky jam and after that the music starts disappearing under a scary and breathtaking sinfony of what is like underwater screaming creatures. That lasts for some minutes, and so a muted electric guitar leads the way gradually from silence to what is one of the most beautiful, energetic and full of emotion musical crescendos a band has ever achieved. Find out the rest of it for yourselves!!!