Well, there are plenty of meanings that this beautiful and excellent poem could be argued to be trying to convey. People have variously argued that this poem is about the beauty of transcendent nature, the simplicity of living or the power of the imagination. I actually think, if we examine the life of the poet who wrote this poem, we can argue that the poem is about the desire to return to a simpler form of nostalgic existence.
Yeats himself possessed this characteristic in the form of wanting to go back to Innisfree, which is a real small island in county Sligo that he used to go to for holidays as a child. This poem was created when Yeats was based in London and walking along Fleet street, which is an incredibly busy and hectic section of this major capital city. As he was walking along, he suddenly related the sound of the fountain to the sound that the water in Sligo lake made. Note how this is refered to in the final stanza of the poem:
I will arise and go now, for always night and day
I hear lake water lapping with low sounds by the shore;
while I stand on the roadway, or on the pavements grey,
I heart it in the deep heart's core.
We see here the central opposition that drives the poem related to the theme of returning to a simpler life. The "pavements grey" and the "roadway" are compared to the "lake water lapping," and it is most definitely the latter that sustains the poem in the "deep heart's core." This poem is above all about the desire to escape the busy present and return to a childlike state of existence where everything is much simpler.