Captivity of a
language measures what portion of the speakers of the language are native. The greater the proportion of native speakers, the more captive the language is.
Languages like
English are learned by millions of
people as a second language, but
Japanese for example is almost entirely
native speakers.
There are about 120m native japanese speakers, and less than 500k fluent non-native speakers.
As the number of speakers goes down, the captivity increases - there are almost no outside speakers of small
natural languages, because learning them isn't very useful. But
constructed languages are totally non-captive, since they have no native speakers.
However, since captive languages don't have many outside speakers, often they are more useful to learn. So if you speak spanish in the
USA it won't do you any good because there are millions of fluent
english/
spanish speakers around, but if you learn
pashtun, you can easily get a job at the
NSA at least.