Cory Booker doesn't have an issues page. So he stands for nothing.
However he did answer some questions from the New York Times and 538.
According to the New York Times, Booker supports single payer but it sounds like he would move to a public option system first, then to single payer.
He's a co-sponsor of the Green New Deal resolution that Alexandria Ocasio-Cortez (AOC) and Ed Markey introduced in their respective chambers.
According to questions from 538, Booker supports marijuana legalization.