Protention is the equivalent of retention with respect to the future. It is the anticipation which is an essential feature of any NOW.
For instance, I can intuit that the present note I am hearing cannot be part of a melody unless some other note will follow it (or a moment of silence if it is the last note). My anticipation is part of my perceiving this note as the note of a melody.
Protention, unlike retention, is not precise. That is, I don't know exactly what note will come next, though most music is pretty predictable. I at least anticipate that it will be in the same key - if it's not I'm suddenly alert and ready for some harmonic changes. Of course if the note sequence C G F were followed by the experience of an elephant, the melody-object I thought I was experiencing would "explode" and I'd have to reevaluate teh meaning of my experience as a melody in the first place. (Think about how protention works in the case of a sentence.)
Just as retention may be misunderstood as recollection, so protention might be misunderstood as an explicit expectation. While reading a detective novel, I might stop and predict "the butler did it." That would be an expectation, not a case of protention. Husserl's point is that even if I never predict or expect in this explicit way, I could not be experiencing what I'm doing as reading a chapter of a detective novel if I didn't have some vague anticipation that there would be future chapters in which the situation I'm now reading about "works out somehow."