The commercial is a comedic play on the idea of celebrities having secret alter egos. Tina Fey plays both the glamorous celebrity "Tina Fey" and her "secret identity" "Meg," a more down-to-earth woman who uses Garnier Nutrisse to maintain her hair color.
It's a bit of a humorous twist on the usual celebrity endorsement, highlighting the accessibility of the product by suggesting even famous people use it.