It makes more sense if you understand that the “thorn” (Þ) is pronounced “th”.

Interestingly, the thorn was in pretty common use until the printing press took off because most of the presses in England were imported from France and Germany, neither of which used the thorn so their typefaces didn’t include one. For a while people used ‘y’ in place of the thorn (hence “ye olde”), but eventually it fell out of use all together