'Hari' represents the manifest universe and life.
'Om' represents the unmanifest and reality - total existence - unseen - invisible
'Tat' means that and
'Sat' means truth.
'Hari Om Tat Sat' means 'That is Truth'. That which I see with my eyes and that which is beyond my eyes are both the same, not different. The creator and the creation are not two. The creator has not created creation, but has manifested or transformed himself into creation.
Adapted from here.