O que significa Riverdale (nome de uma série da Netflix)?

Nada. É um nome próprio, da cidade onde se baseia a série.

Há um sentido.

River: rio
Dale: vale (planície à beira de um rio, depressão alongada cavada por rio ou geleira)

Literalmente, Riverdale significa Vale do rio. As imagens iniciais da série lembram este cenário 'de vale e rio.'

I can relate to that situation. Sometime ago, when I was at the beggining stage of learning I was reading an item about Ozzy Osbourne, from the caption I had figured that Ozzy was a rocker guy, I got puzzled with 'Osbourne'.
Then I looked up the word since in a dictionary; I thought it was a verb, nothing showed up. Only after quite a while that I learned that it was part of his name!
Unfortunately English Experts didn´t exist at that time...