What has happened is that hot water dissolves the powder into tiny particles that chain together. These particles are so small that you can't see them, but they are floating in the water.
When each small chain cools down, it begins to act like a magnet. It can't pick up nails or tacks, but it can attract water, and that's exactly what happens: more and more drops of water cling to each small chain until all the contents in a container are gelatinized.
When gelatin is cooled, it hardens because these chains contain water droplets tightly together. But if the gelatine is reheated, the water is released from each chain and the mixture becomes a liquid again.
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