A PADD showing Kirk and Spock from the "TOS era"
TOS era was a colloquial reference to the 2260s made by Commander Jack Ransom of the USS Cerritos.
He said that it referred to "'those old scientists' – You know, Spock, Scotty, those guys. Seems like they were stumbling on crazy new aliens every week back then." (LD: "No Small Parts")
Ensign Brad Boimler used the term when he and Ensign Beckett Mariner returned to their own time after visiting the USS Enterprise in 2259. (SNW: "Those Old Scientists")
Star Trek: The Original Series is often referred to as "TOS" by fans. This is a meta reference to the fact that The Original Series characters would encounter new aliens on each new show, which aired weekly.
Additionally, the picture of Spock and James T. Kirk is drawn in Star Trek: The Animated Series style.