The final hypothesis of Krashen’s basic theory of language acquisition is the Affective-Filter hypothesis which “states that attitudinal variables relating to success in second language acquisition generally relate directly to language acquisition but not necessarily to language learning.” 
This is a way of recognising that there are other factors, a Filter, that influence how well a person acquires language when presented with comprehensible input. For instance, a positive attitude towards speakers of the language will lower the filter, while a negative attitude will increase it. A need to function in the language will generally lower the filter, but anxiety and early-forced-production can raise the filter.
A function of the Affective-Filter is that test for aptitude in the area of language tend to test language learning rather than language acquisition. These are not necessarily correlated. For example, a test question that gives you a number of language patterns and their meanings, and allows you to inductively work out the differences, generally reveals a person’s ability to inductively analyse grammar, not their potential to acquire language.
This impacts the general snobbery of classical language students and teachers, who think that only the more capable will succeed in the classical languages. that certainly is true if what we mean by ‘succeed’ is language learning. Those who genuinely thrive on Grammar-Translation are in the minority (and I’m probably one of them which is why I got so far but also why I write about this stuff); that minority though is terribly reinforcing.
No, the evidence of the world at large is that most people, given exposure and enough reason, will learn to function communicatively in a second, third, seventh language even. A fellow student in my graduate program from Africa has about seven languages to his credit, most of which he can function communicatively in. This is not unusual outside the west, and especially outside monoglot-English-language contexts.
Its second consequence is in shaping language courses. Not only should they aim at providing meaningful and interesting comprehensible input as messages, but they should aim to provide a context which works to lower the Affective Filter. In this manner, hypothesis five speaks to language teachers as facilitators of language acquisition, not imparters of conceptualisations.
This concludes my short series looking at the core of Krashen’s work on Second Language Acquisition. I hope it’s been enlightening to you, revealed some of the theoretical framework out of which I operate, and challenged some of your assumptions and methods. Given that this stuff is on my mind at the moment, I imagine you’ll get a few more posts on the topic area in days to come.
For more information on Krashen, the best thing to do is go to his website. He has several books readable online that outline this theory in more depth. The address is: http://sdkrashen.com/
Earlier posts in this series:
 Krashen, S & Terrell, T. The natural approach : language acquisition in the classroom Oxford [Oxfordshire] ; New York : Pergamon Press ; San Francisco : Alemany Press, 1983. p37-8