Strong forms occur in stress-timed languages such as English when the word is fully stressed. This full stressing of a word which is normally unstressed will frequently change significantly the information which the speaker is conveying.
For instance, if a speaker says "I would like fish 'n' chips" then there is nothing more to the sentence than the simple request. However, if they were to say "I would like fish and chips" then something more is being expressed which can only really be understood from the context. Does the shop usually only sell fish? Has the purchaser been there before and only received a few chips? Without more context it is impossible to say by a simple examination of the grammar and the vocabulary.
 See also