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.
We can infer that the shape of Earth's orbit around the sun is elliptical.