What is this poem about?

Long River = another name for the Yangtze river.

the foam of the waves breaking denote the passage of time,

The green hills and the sunsets denote the constancy of things to be compared with the ever changing flux of human events.  The rest of the poem reinforces this contancy of things and the ethereal nature of human lives.  So take not life too seriously as things whether good or bad will pass and a story they become for the future generations.

What is this poem about?

Sorry for my broken English. It basically refers to the kind of life which clings to nothing. All the things, success or failure, come and go, never last long. So we should just enjoy what we have currently. Most of Chinese traditional poems shared this kind of subject, but I think most of the poets who wrote these poems never got to this state of mind, they just pretended to be tough. However it's zenic and beautiful, one of my favorite poems.

This poem 臨江仙 is from Yang Shen, a Ming Dynasty poet whose poem was added on to the preface to the Romance of the Three Kingdoms. Basically it just a poem to reminisce the turbulent times portrayed in the story, and eventually when everything is said and done, it was as fleeting as the roaring river, and only lives on as small talk and legend. (Celia did a much neater job of summarising the poem. If I could upvote her, I would).

I have to add though, Yang Shen didn't write it for the purposes of the story. It was until much later (maybe mid Qing Dynasty) before someone decided it was a good idea to pop it in.