This questions refers to the structure of the play. Instead of presenting the plot in a straightforward manner, Miller has chosen a nonlinear presentation which combines Willy's present day reality along with flashbacks to fifteen years earlier and his imagined conversations with his brother Ben. Each of these segments brings new meaning to Willy's character and the way the reader relates to him.
Luckily, Miller provides some auditory cues as to the changes in time period. Ben has his own musical cue, and varying types of music alert the audience (or the readers via stage directions) as to impending flashbacks or scene changes.
On a content level, the depth and degree to which Willy lapses into his imaginary conversations or seeks the happier days in his flashbacks depends upon his level of anxiety or mental deterioration. The sight of stockings produces guilt in Willy which conjures up The Woman. Money issues tend to produce Ben, and as his fight with Biff in the restaurant escalates, so does the confusion for the reader, for at this point, the reader must muddle through both imagination and flashback at the same time.
These nonlinear presentations help to round out Willy's character and help the reader sort out his life.