What does this percentile thing actually mean? I'm guessing the aim is to be in the same percentile for length and weight?

My son was measured this morning and I checked him on the charts after getting home and he is a skinny little thing! I knew that but now I just want to know it's okay!

According to the WHO BFing charts he is just under the 50th percentile for his weight but he's on the 85th percentile for his length. That seems a bit off to me.