The poem is about a kite. Kite is made out of paper and is later tied to a string.
A paper kite with a string
The colourful kite soars high in the sky, and when it is a new kite, it looks beautiful. The line "how bright on the blue" means how bright in the sky. Here, the poet calls the sky blue as the sky is blue in colour. It can also mean that the sky is clear without clouds.
A colourful kite in the blue sky
Generally, the kite does not stay straight; it sways like waves. It also doesn’t fly at the same height: sometimes it flies high and occasionally low. But then, with the help of its tails, it can soar even higher.
Tail helps the kite soar higher
The kite looks like a ship sailing on the high tides. Here the soaring of the kite is compared to the sailing of the ship.
A kite is compared to a ship

As the wind pushes the ship, it sways and moves through the water. Similarly, the kite climbs higher as the wind blows.
Later, when the wind drops, the kite shifts into a rest mode.
As the wind falls, the string of the kite gets loose. So, they will wind the string back and will again run with it until there is another breeze.
A girl running with her kite
Once the breeze starts, the kite will be blown back into the sky. So, the colourful kite will once again be soaring beautifully in the blue sky.
But, all of a sudden, when the string is cut off, the kite will drop itself on a tree.
A kite stuck on a tree
As a result, the kite will get torn as the kite is made of paper.
A torn kite