For pronunciation, one resource is https://en.wikipedia.org/wiki/Hungarian_phonology#Vowels

That particular diacritic mark is a double accent.

https://en.wikipedia.org/wiki/Double_acute_accent#Hungarian

> "In Hungarian, the double acute is thought of as the letter having both an umlaut and an acute accent"

It's a long o-umlaut. Pronounced like "err" (thinking) or a stretched out version of "ea" from earth.

Incidentally, Erdős in Hungarian means Forester.