"Stop by" is used idiomatically as a verb meaning "visit briefly", e.g. "stop by Bill's house" means "visit Bill's house briefly". (You can't really analyse "by" separately; literally, it would mean "stop next to", but that's not what is understood.)

Meanwhile, "back" here refers to returning, e.g. "Go back to Bill's house" means "go to Bill's house again, which you've been to before".

The sentence is missing a mention of any place, so it is assumed by context to be "here", most likely the speaker's house or office.

So, "Stop back by" is a short and colloquial way to say "Please visit me here again in future".

"Stop by" is used idiomatically as a phrasal verb meaning "visit briefly", e.g. "stop by Bill's house" means "visit Bill's house briefly". (You can't really analyse "by" separately; literally, it would mean "stop next to", but that's not what is understood.)

Meanwhile, "back" here refers to returning, e.g. "Go back to Bill's house" means "go to Bill's house again, which you've been to before".

The sentence is missing a mention of any place, so it is assumed by context to be "here", most likely the speaker's house or office.

So, "Stop back by" is a short and colloquial way to say "Please visit me here again in future".

"Stop by" is used idiomatically as a verb meaning "visit briefly", e.g. "stop by Bill's house" means "visit Bill's house briefly".

Meanwhile, "back" here refers to returning, e.g. "Go back to Bill's house" means "go to Bill's house again, which you've been to before".

The sentence is missing a mention of any place, so it is assumed by context to be "here", most likely the speaker's house or office.

So, "Stop back by" is a short and colloquial way to say "Please visit me here again in future".

"Stop by" is used idiomatically as a verb meaning "visit briefly", e.g. "stop by Bill's house" means "visit Bill's house briefly". (You can't really analyse "by" separately; literally, it would mean "stop next to", but that's not what is understood.)

Meanwhile, "back" here refers to returning, e.g. "Go back to Bill's house" means "go to Bill's house again, which you've been to before".

The sentence is missing a mention of any place, so it is assumed by context to be "here", most likely the speaker's house or office.

So, "Stop back by" is a short and colloquial way to say "Please visit me here again in future".