Nathan was late being born and so was being monitored by this machine
The doctors saw that Nathan was not going to be born naturally and was becoming distressed so they decided to perform a Caesarean Section.
A Caesarean Section is when the doctors cut open Nathan's Mum's tummy and delivered him that way.
It is said to be named after the Roman Emporer Julius Caesar, but that cannot be true because the baby's mother always died in those days and since his mother lived well past his birth he cannot have had a Caesarean!
Nowadays it is a very safe way of being born and is even preferred where the baby might become distressed by a normal birth.