Alma brought her 6-month-old son to the doctor for a well-baby visit. The doctor says that her son's weight is in the 20th percentile. What does this mean?

This means that her son is small compared to other 6 month old children.

A percentile is a measure between 0 and 100 that compares children of similar age in order to know if a particular child is growing well. The higher the percentile number, the bigger the child is compared to other children of same age. If a child is in the 50th percentile for height, it means the child is in the middle of the pack.