Phillip Halpert is the second child of Jim and Pam Halpert.
He was born off-screen and wasn't seen on air until a few months after his birth. He was assumed to be born around December 5th or so based on Pam's online blog.
Phillip is named after Pam's grandfather. He is the younger brother of Cece Halpert who often calls him Pull Up instead of Phillip.
While Jenna Fischer, the actress that played Pam Halpert, was pregnant in real life during this storyline, the second Halpert child was already planned to happen during this time.
Phillip is also the name of Angela's baby that she had in the episode "Jury Duty". Phillip was Jim and Pam's second baby. Pam went on maternity leave in "Gettysburg" and came back in "Jury Duty".