Theory:

He gives his harness bells a shake
To ask if there is some mistake.
The only other sound’s the sweep
Of easy wind and downy flake.
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Explanation:
 
Little horse was feeling something odd. The poet has a belt tied on the neck of the horse to control it, and he gives them a little shake  to get his master's attention. The only other sounds are of a slight wind and falling snow. It was so quiet.
 
Meanings of difficult words:
 
Numbers
Word
Meaning
1.
Harness bells
A belt tied on the back of the horse.
2.
Sweep
Glide.
3.
Downy flakeSoft feathery small pieces of snow falling from the sky.
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.