The baby boom was the period after World War II in the United States when more babies than usual were born.
A baby boom can refer to any increase in the population, but it usually refers to the time just after World War II when the population went through the roof in the United States. There are more baby boomers — as these kids were called— than the generations before and after them. It can help you remember this term if you know that a boom is an increase or spike in something, like a boom in business when profits are high.