It might mark a boundary between two different pronunciations (grass to rhyme with crass and grass to rhyme with farce, perhaps); or two different meanings for the same word (canny can mean either kind or knowing, depending on whether you're in Northern England or Scotland); different ways of saying the letter r; or even two quite different languages.
Here's a map of Germany from Wikipedia.
Public Domain, https://commons.wikimedia.org/w/index.php?curid=1008094
This map shows the division of High German into Upper and Central German (green and blue, respectively) as distinguished from Low Franconian, and Low German, which is shown in yellow. The black lines show the famous Benrath and Speyer lines which divides the languages (which just goes to show what an interesting word famous is).
Of course what we really need is a three-dimensional map that shows us the effect of time, as well. Then we could see how everyone stopped saying dear meaning expensive, and when the word often had a sounded letter t.*
It could show us what effect the advent of literacy, radio and TV had on language, too.
Whether wisdom would emerge from such a device I do not know. Probably we'd be left with a puzzle that makes a Rubik's cube look like a one-piece jigsaw.
Still, that's what academics are for, isn't it.
Word To Use Today isogloss. Iso- comes from the Greek isos, which means equal or similar; and glōssa is Greek for dialect or tongue.
As the intelligent reader will have spotted, an isogloss shows, not similarities, but differences, and this is why some people point out that it would be more logical to call it a heterogloss. (Hetero means other.)
But then, what has logic got to do with language, eh?
*In the mid-twentieth century, I'd guess. It was part of the speak-as-you-spell movement.