I'll start by describing the experience of understanding aspoken sentence: "the cat is on the mat." If we accepted thepunctual dogma, then we have a dilemma. If we can hear only oneword at a time, the sentence is incomprehensible. On the otherhand, if, when we get to the last word, "mat," we recollect all theprevious words simultaneously, we just have a jumble we can make nosense of. Presumably if we recollect all the words at the end, wewould have to run over them again in their temporal order to makessense of them. But what does "run over them again" mean? If itmeans relistening (in our imagination this time) to each in turn,we still have the same problem when we get to the end of thesentence a second time!
Let us forget the dogma and just describe what we are given.When we understand a sentence, each previous word is retained andremains present, but remains present in a manner that does notconfuse it with the current word, that is, it is given toconsciousness as past. All these past words, however, are not givenas homogeneously past, for then they would be just a senselessjumble. They maintain the order in which they were originallygiven. That is, when we get to "mat," "the" is present as theimmediate past which is sinking away, and "on" is present to us asthe word which was the immediate past when "the" was the currentword, and so on. The past remains present without becoming actuallycurrent. Don't ask how this is possible; just describe it!
Husserl labels this phenomenon "retention." Retention is the mode in which the past is present to consciousness. (Of course, there is also a complementary phenomenon of protention in which the likely future is anticipated, but for the sake of simplicity I will ignore this.)