All this is saying is that continued fraction has the form,

[A;B,C,D,E,...]

Where,

B,C,D,E,... are all positive integers.

While,

'A' can be any integer (also negative).

The reason why is that if we let one of the B,C,D,E,... be zero it might let to division by zero .

The rule with these continued fractions is that they are between 0 and 1.

Meaning any continued fraction you chose with the first term being zero, its values will not exceede 1.