"Life can only be understood backwards, but it must be lived forwards." This famous saying of Kierkegaard may seem obviously true: the future is indeterminate, so 'my life in the future' is not something definite that I can understand now; by contrast, my life in the past is fixed, but it is gone now and I can only live in the present, moving forwards into the future.
However, I can have a plan for my future and understand what my life should be like according to the plan. This is, in a sense, understanding life forwards. On the other hand, I cannot live without being affected by my memories of the past. If my memories are so powerful that they dictate how I live my life now, then I am in a sense living in the past, living my life backwards.
There is indeed an entry in Kierkegaard's journal in which he remarks that "life must be understood backwards", but what he says next is a qualification of the remark:
And if one thinks over the proposition it becomes more and more evident that life can never really be understood in time simply because at no particular moment can I find the necessary resting-place from which to understand it --- backwards.
There is no resting-place because my life changes constantly for as long as I live. How I understand my life so far depends on how I see things and how I live my life now, but both of these can change (and usually not as a result of my own decision), and my understanding of my life will change accordingly. My understanding of my life is thus always tentative.