Theory:

You may scare a rabbit
Who is sitting very still;
Though at first you may not see him,
When he hops you will.
Explanation:
 
Remember the poet asking us to walk through the meadows slowly? The poet had described in the previous stanza as to how to spot a butterfly by being quiet. In this stanza, he talks about how one could spot beautiful rabbits in the meadows.
 
Rabbits have the habit of sitting still and quiet. It is hard to spot a rabbit as they can hide itself well among the bushes. You may have to move slowly and look around carefully to see one.
 
animal-1845263_640.jpg
A rabbit sitting very still
  
On the other hand, if you are not quiet, you might scare it away. Though you would miss the chance of observing it from a closer distance, you might at least be able to see it hop.
 
Or, as the poet says, you may realise its presence only when it hops and runs away.
 
rabbit-2405552_640.jpg
A rabbit that hops
 
Meanings of difficult words from the stanza:
 
Sl. No.
Words
Meanings
1
Hops      Move by jumping with two or all feet at once
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Meadow Surprises: Lois Brandt Phillips (pp. 123-124). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.