Keyline design is a way of designing things, like farms or gardens, so that they use water in the best way possible. It helps to make sure that water goes where it is needed most and doesn't get wasted.
Imagine you have a farm with a big hill in the middle. When rain falls, it can either run down the hill and get lost or soak into the ground where it is needed. With keyline design, you would dig little trenches, sort of like small canals, along the hill in a special pattern called a "keyline." These trenches would help to guide the water down to the lower parts of the hill where it could be used to grow things like crops or trees.
It's like giving water a map to follow, so it doesn't go to waste or cause problems like erosion. With keyline design, we can grow things more efficiently and sustainably while also conserving water resources.