When you prune a plant, you are removing dead growth and removing selected branches, flowers, etc., in order to promote healthy growth, improve the shape of the plant, and encouraging an increase in flowering and fruit.
Synaptic Pruning
Synaptic Pruning is similar. During childhood the brain is quickly growing and developing. At birth the brain is approximately 1/4 the size of an adult brain - with billions of neurons. It makes more than a million neural connections per second. By 1 year of age, it has doubled in size and by the time we are 5-6 years old, our brain is 90% the size of an adult brain. Synaptic Pruning is when the brain eliminates synaptic connections that are not being used in order to enhance efficiency and functionality. It is believed that this is a crucial part of cognitive development and plays a part in memory and learning. You can learn more (and watch a video) here.