Imagine a plant growing in your backyard. It needs a warm and cozy environment to grow, just like how you need a warm blanket in winter to snuggle up.
Similarly, a hotbed is a warm and cozy bed for plants. It's like a small space filled with heat that plants need to grow quickly. In a hotbed, the soil is warmed up artificially so that it becomes a perfect place for seeds to grow into healthy plants.
Usually, gardeners create a hotbed by placing a layer of fresh manure covered by a layer of soil. The manure (which is poop from animals) produces heat as it decomposes, and that heat helps to warm up the layer of soil above it.
When the soil is warm, the gardeners can plant seeds or transplant small seedlings into the hotbed. This warmth helps the seeds to rapidly sprout roots and grow into healthy plants.
Hotbeds allow plants to grow faster and better than they would in the colder environment outside of the hotbed. And just like how you would love to cozy up in a warm bed, plants also love to snuggle up in a hotbed to grow big and healthy!