Sensitive periods in a child's life are like windows of opportunities a child gets to learn different skills.
The child is extra sensitive to the stimuli received to learn a particular skill during these periods. If the child does not get the necessary stimuli during this period the opportunity to acquire the skill with ease is missed forever. It is important to understand this and as adults we must be aware of such periods in the child’s development, so that we can provide help to the child when needed.
"There are limits to the brain’s ability to create it. Time limits. Called "critical periods", they are windows of opportunities that nature slams shut, one by one, with every candle on the child’s birthday cake."