What do the words "autumn" and 'autumn leaves" mean in this ...

Ирина

What do the words "autumn" and 'autumn leaves" mean in this song?
 
Leaves of autumnI've never known what love could be, Your picture's smiling back at me. You've changed my world, but had to go away, And now my heart lives in the yesterday, And I am surrounded with the leaves of autumn.
 
RefrainThe whole world is covered by autumn leaves, They drift by the window, they fall from above, They'll cross all the continents and the seas And whisper about my love.
 
This autumn took my love away, It seems we parted just today. But days and weeks are going slowly by, I start forgetting how you smile and cry, I shout your name into the rain of autumn.
 
The world has turned the shade of grey,It is the price I have to pay, For all the happiness you brought to me. For learning how to be unlonely, My heart is burning with the fires of autumn.
 
The whole world's taking us apart, Friends say I shouldn't even start. But I know there will be tomorrow, We'll share our secrets, dreams and sorrows, I write my letters on the leaves of autumn.