The Wikipedia entry is simple: Coercion (linguistics) - Wikipedia. The definition is "a mismatch of temporality between lexical items", with an example like: "the tiger jumped for an hour," where the prepositional phrase "for an hour" coerces the lexical meaning of "jumped" to be iterative across the entire duration, instead of having occurred only once.
Asking about aspectual coercion particularly in relation to the present perfect is conspicuous. As far as I know, coercion of the perfect only applies to English and the Scandinavian languages.
@daniel895: I suggest that you first read up on aspectual coercion in relation to aktionsart and then you can move on to the different flavours of the perfect in English. Maybe it will be clearer to you then.