Theory:

Meadows have surprises,
You can find them if you look;
Walk softly through the velvet grass,
And listen by the brook.
Explanation:
 
The poet talks about the surprises in nature and meadows in particular. Meadows are places where you could find a lot of grass and wildflowers.
 
path-832031_640.jpg
Meadows found on the either side of the pathway
 
A meadow and garden contain the same things. They both have trees, flowers, and plants. However, while a garden is grown and tended to, meadows are purely natural. They contain wildflowers and grasses instead of utility plants. The plants in the meadows are not planted by the humans.
 
garden.jpeg
Garden
  
Back to the poem, the poet says that one could find many surprises in the meadows, as long as you know how and where to look for it.
 
If you want to discover the surprises, you should walk softly through the grass. When you reach the little river running by the meadows, the poet asks us to stop and listen.
 
stream-5627504_640.jpg
A brook (river) in the middle of a meadow
 
The poet asks us to walk softly because, otherwise, we might scare the inhabitants of the meadows away.
 
The poet also describes the grass as something soft like a velvet. The phrase "velvet grass" is a metaphor. The texture of the grass is compared to the softness of velvet here.
 
prato-3715835_640.jpg
Grass which is as soft as velvet
 
Meanings of difficult words from the stanzas:
 
Sl. No.
Words
Meanings
1
MeadowsA piece of land that is covered or mostly covered with grass
2
BrookA small, narrow river
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.