Theory:

And eats up insects for his food.
So when he wiggles in the grass
I’ll stand aside and watch him pass,
And tell myself, “There’s no mistake,
It’s just a harmless garden snake!”
Explanation:
 
The speaker sees a garden snake and gets scared. He thinks that all snakes are supposedly dangerous. He does not know to differentiate the good ones from the harmful ones. His mother, having some expertise in that area, advises him that the particular snake was a good one. The speaker and the readers are left to wonder if good and snake is an oxymoron. An oxymoron is when two completely opposite words are put together to make sense. For example, 'pretty ugly' is an oxymoron. But the speaker's mother explains the logic behind referring to the snake as good.
 
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He runs away seeing the snake
 
A food chain is a link where organisms are related by the food they eat. Every organism in the world has a purpose for its existence. Likewise, the garden snake consumes insects and flies as its food. This helps to get rid of pests and insects. If not for the existence of the garden snake, then the garden would be filled with insects, which in turn would spread deadly diseases. The speaker understands the concept after his mother explains it. He therefore does not run away or try to harm it.
 
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Food Chain
 
The garden snake moves around on the grass in the garden. It does not attack anyone as long as it is not disturbed. It stays in its own zone and serves its purpose. The poet meanwhile stands aside and allows it to pass peacefully. The line stresses on the concept of co-existing peacefully with our eco-system. The poet tells himself that there is no regret in letting the snake live as it is a mere harmless being trying to survive in this world, just like the other creatures.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
WiggleMove around in different directions
2
Eco-systemA community of organisms in a particular environment
3
ConsumeTo eat
4
ExpertiseExpert in a particular field
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Garden Snake: Muriel L. Sonne (pp. 137-138). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.