they are made of sand
This poem is about doing something without the realization that it will be undone. This idea could apply to many situations. As an example, imagine someone starts a business. They spend a great deal of effort setting it up. They make it exactly how they want. In a year though, the business the fails. The person thought they were building something that would last. Instead, they built something that disappeared.
This particular poem was based off of the idea of weight loss. Sometimes when a person loses weight, they keep it off for a while, but they gain it back. When the person lost the weight they thought it was permanent. Instead, things went back to the way they were. This idea was represented symbolically by the idea of person building what they thought was a stone castle, but it turned out to be a sand castle. A sand castle is a temporary structure that will quickly disappear.
The idea of the poem is to contrast the idea of perceived permanence, with the reality of temporariness.