Theory:

The woods are lovely, dark and deep.
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
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Explanation:
  
The poet admits to having a strong desire to have or do something for the dark woods, but he tells us he's got more things to do, people to see and places to visit. He's got a long way to go before he can rest or sleep, so he had better get going.
  
Meanings of difficult words:
 
Numbers
Word
Meanings
1.
Promise
Vow.
2.
MilesA long way.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. Stopping by the woods on a snowy evening - Robert Frost (pp. 146-147). Published by the Tamil Nadu Textbook and Educational Services Corporation.