Theory:

You may see a butterfly
Rest upon a buttercup
And unfold its drinking straws
To sip the nectar up.
Explanation:
 
While walking through the meadows, you may come across various surprises. One of those surprises are butterflies. We have seen butterflies, and have obviously heard about it. We have seen some of them flutter around us.

garden-19830_640.jpg
A butterfly

However, the question is, have you observed a butterfly sucking nectar from a flower? Or have you seen the sucking tube known as proboscis through which the butterflies suck the nectar?
 
It is hard to watch a butterfly sucking nectar because it flutters away the moment we get anywhere near it.

Hence, you should walk softly through the meadows, and when you do that, you will able to see those butterflies in action.

The poet says that one could discover butterflies resting upon the wildflowers such as buttercups. And if you observe closely, you could see it opening the sucking tube to draw nectar from a flower.
 
peacock-butterfly-3482707_640.jpg
A butterfly sucking up nectar through its sucking tube
 
The poet also makes a hidden comparison between the sucking tube (proboscis) and a drinking straw. This is an instance a metaphor.
 
Meanings of difficult words from the stanza:
 
Sl. No.
Words
Meanings
1
ButtercupA small, bright yellow cup-shaped wild flower which is common in grassland and as a garden weed
2
NectarA sugary fluid secreted within flowers, and this is eventually collected by bees to make into honey
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.