Further is the older of the two, with farther originating from it as a variant in Middle English. For much of their history the words have been used interchangeably. As adverbs, they still are interchangeable when applied to distance (whether spatial, temporal, or metaphorical).

fur•ther /ˈfɜrðɚ/ comparative adv. and adj. of far with superlative fur•thest, v.

In Middle English, it was spelled “further,” and it wasn’t until the 17th century that “farther” emerged as a variant.

The simple definitions of further and farther are both terms that mean more far, which is a grammatically incorrect phrase (that’s why we use further/farther) but does offer a succinct explanation of the definitions.