Theory:

I will arise and go now, for always night and day
I hear the lake water lapping with low sounds by the shore;
While I stand on the roadway, or on the pavements grey,
I hear it in the deep heart’s core.
Explanation:
 
The speaker begins the third stanza with the refrain "I will arise and go now", reinstating his desire to travel to Innisfree.
 
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Innisfree is where the speaker wishes to be
 
However, the rest of the stanza reveals that Innisfree is not merely a dream but rather a desperate need. His memories of Innisfree haunt him "night and day". He could always hear in his mind the sound of the lake water lapping against the shore. The speaker calls the sound "low" because Innisfree is a distant memory. Though the memory is distant and faint, his desire to relive the days spent at Innisfree is powerful and desperate.
 
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Water lapping against the shore

The third line of the stanza suggests that the speaker is tired of living in the city. He could hear the call from the island deep in his heart while standing on the roadway. The words roadway and pavements imply city here.

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He stands on the grey pavements

The phrase "pavements grey" stands in contrast to the colourful Isle of Innisfree. The phrase reveals the speaker's feelings towards city life. Life in the city is dull, concrete, and grey, while life at Innisfree is beautiful, lively, and meaningful.
 
Meanings of difficult words:
 
S.No
Words
Meanings
1
Lapping(Of waves) to hit something gently, producing quiet sounds
2
DesperateNeeding or wanting something very much
3
HauntBe constantly and spontaneously present in the mind
4
ConcreteA very hard building material made by mixing together cement, sand, small stones, and water
Reference:
National Council of Educational Research and Training (2006). Beehive. The Lake Isle of Innisfree- W. B. Yeats (pp 54). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.