Time slot

President Barack Obama shared some words of wisdom on Saturday, saying there a few things in life harder to find and more important to keep than love.
"Well, love and a birth certificate," Obama quipped at the White House Correspondents' Association dinner, poking fun at those who question his place of birth. "I happen to know that my approval ratings are still very high in the country of my birth." Obama was born in Hawaii, but birthers question whether he was born overseas.
Obama also jabbed Jay Leno, the comedian headlining the dinner. Obama dinged Leno as "the only person whose ratings fell more than mine."
He said he was glad he spoke before the television host, "because we have all seen what happens when somebody takes the time slot after Leno." Comic Conan O'Brien left NBC after his stint hosting "The Tonight Show" following Leno didn't work out.

Please explain to me the emboldened parts.

Source : http://www.ndtv.com/news/world/obama-pokes-fun-at-mccains-maverick-claim-22534.php