It says 'if it does not refer to a previous word'. In this case, den does refer to a previous word, that word is just not in this sentence, but it has been mentioned in the context or it's understood what it is.
The 'tips and hints' refers to expressions like Det är en bok where 'det' does not refer to the book, but is a placeholder subject to say 'the thing I'm now talking about'.
"Det" is used as a dummy pronoun or when you refer to an 'ett' word.
A dummy pronoun doesn't refer to anything, but is still required to make sense. For example, the 'it' in 'it is raining' is a dummy pronoun because it isn't referring to anything. It's there only because 'is raining' makes no sense on its own.
It is raining - Det regnar
I see the house. Do you see it? - Jag ser huset. Ser du det? ('det' is used because it refers to the house, which is an 'ett' word. If the noun were an 'en' word, you would use 'den')
This is where knowing the gender of nouns is extremely important. "Den" refers to an "en" noun (en flicka, en pojke, and so on), whereas "det" is used for "ett" nouns or when the noun isn't known and hasn't yet been mentioned. It is also used for sentences like "it is raining".
Den - I don't see my newspaper. Do you see it? (Jag ser inte min tidning. Ser du den?
Det - My cat likes meat, but I don't like it. (Min katt tycker om kött, men jag tycker om inte det)