This is the 551st most frequent German word.
nämlich
"Namely" or "that is to say."
In this sentence, "nämlich" is used to specify or clarify the kind of flowers.
Ich habe schöne Blumen gekauft, nämlich Rosen.
I bought beautiful flowers, namely roses.
Here, "nämlich" provides the reason or explanation for why Maria wasn't at home.
Maria war nicht zuhause, sie war nämlich noch im Büro.
Maria wasn't at home, as she was still at the office.
This usage of "nämlich" serves to provide additional detail or specification about the time.