Protention is the equivalent of retention with respect to thefuture. It is the anticipation which is an essential feature ofany NOW.
For instance, I can intuit that the present note I am hearingcannot be part of a melody unless some other note will follow it(or a moment of silence if it is the last note). My anticipationis part of my perceiving this note as the note of a melody.
Protention, unlike retention, is not precise. That is, Idon't know exactly what note will come next, though most music ispretty predictable. I at least anticipate that it will be in thesame key - if it's not I'm suddenly alert and ready for someharmonic changes. Of course if the note sequence C G F werefollowed by the experience of an elephant, the melody-object Ithought I was experiencing would "explode" and I'd have toreevaluate teh meaning of my experience as a melody in the firstplace. (Think about how protention works in the case of asentence.)
Just as retention may be misunderstood as recollection, soprotention might be misunderstood as an explicit expectation. While reading a detective novel, I might stop and predict "thebutler did it." That would be an expectation, not a case ofprotention. Husserl's point is that even if I never predict orexpect in this explicit way, I could not be experiencing what I'mdoing as reading a chapter of a detective novel if I didn't havesome vague anticipation that there would be future chapters inwhich the situation I'm now reading about "works out somehow."