"Usually the screen angle for Yellow is such as to retain a 15° offset from as many other colors as possible"

In other words:
in CMYK print, the screen angle (inclinazione di retino) for each ink has to be different from that of other inks (so as not to cover each other up, but to produce dots of inks close enough together to give the isslusion of a full-colour image). Also, to prevent noticeable color dot patterns (moiré) they say to keep a 15° offset (difference) between Yellow and as many other inks as possible (basically 2 out of the 3 other colors) - it will be mathematically impossible to have a 15° offset between yellow and each one of the other inks. So: if the screen angle for Yellow is set at 90°, the screen angles for Black and Cyan will be 105° and 75° respectively.

Actually, ISO standards say something slightly different...
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ISO standards (Figure 10, page 32) for angle sets in printing define what to do with the fourth angle in the four-color process.
ISO Rule 4.2.3 states four-color process angles should be set with the three dark colors (cyan, magenta and black) 30 degrees apart and yellow should be 15 degrees from another color. The “other colors” that yellow should normally be 15º apart from are the black and cyan.
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(from the link below, with nice illustrations)

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Note added at 1 hr (2006-06-10 14:00:18 GMT)
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another useful link:
http://assigneddesign.com/tips_for_printing_a_good_book.htm