Experience plays a very important part when it comes to the development of the human brain in infants and babies. This is because experience helps to build connections, synapses, within the brain, which allow it to develop further and to grow.
Learning takes place mainly through experiences in babies, as babies develop behavioral patterns mostly through conditioning. Pavlov demonstrated this very famously through his experiment with a dog—after having heard a bell being rung before being given food several times, the dog had learned to connect the sound of the bell with the approaching of food. This led to the dog starting to salivate merely upon hearing the bell, without even needing to see the food. The same applies with babies, as babies' brains develop by making connections and links similar to the one Pavlov observed in his dog.
For example, a baby will initially appear to make a smile by simply experimenting with moving its facial features and copying the facial expressions of those around him. However, soon the baby will realize that once it smiles at someone, this person will smile back and perhaps even praise the baby or stroke it. In order to create this positive experience again, the baby will continue to attempt to smile in order to create this same positive reaction again. This is what's called positive conditioning.
Another example to show you how experience helps the development of the brain in babies is the development of speech. Over time, a baby will come to realize that the word milk is said every time the mum feeds the baby milk. Eventually, the baby will be able to copy this sound and utter it every time it is hungry, as the baby's brain will have worked out that the sound of the word milk refers to the white liquid that removes the baby's feeling of hunger if the baby drinks it. Therefore, as the baby grows up, it will learn to ask for milk itself when it is hungry (or at least produce a sound similar to this word, in an attempt to try and copy what it has heard from its parents).