That's an old letter called "thorn"! It stands for the "th" sound in "thing" and "oath", but not the "th" sound in "this" or "loathe".
We used it once, but not anymore, which lands it in a category of things Devine Lu Linvega (npub1z8r…mrq6) takes great interest in 😄
https://en.wikipedia.org/wiki/Thorn_(letter)
Edit: nowadays it's still used in Iceland. ❄️🌋❄️