Circuit split occurs when two different circuit courts rule differently on a given issues. Since circuit courts are limited by area jurisdiction they do not overlap and therefore a differing ruling from different courts can essentially result in different federal law in different parts of US. Circuit courts are also not bound by precedents of other circuits.

There are 12 (well technically 13) Circuit Courts of Appeals in US:

1. First Circuit: Maine, Massachusetts, New Hampshire, Rhode Island, Puerto Rico
2. Second Circuit: Connecticut, New York, Vermont
3. Third Circuit: Delaware, New Jersey, Pennsylvania, Virgin Islands
4. Fourth Circuit: Maryland, North Carolina, South Carolina, Virginia, West Virginia
5. Fifth Circuit: Louisiana, Mississippi, Texas
6. Sixth Circuit: Kentucky, Michigan, Ohio, Tennessee
7. Seventh Circuit: Illinois, Indiana, Wisconsin
8. Eighth Circuit: Arkansas, Iowa, Minnesota, Missouri, Nebraska, North Dakota, South Dakota
9. Ninth Circuit: Alaska, Arizona, California, Guam, Hawaii, Idaho, Montana, Nevada, Northern Mariana Islands, Oregon, Washington
10. Tenth Circuit: Colorado, Kansas, New Mexico, Oklahoma, Utah, Wyoming
11. Eleventh Circuit: Alabama, Florida, Georgia
12. DC Circuit: District of Columbia
13. (Federal Circuit - This one is weird, it hears cases based upon subject matter like patent law, rather than geographic location. Precedents are binding to entire country. Due to the limited jurisdiction this one can't hear a DACA case.)

So out of the above 12 circuits every one can hear a lawsuit revolving around DACA or DACA repeal and rule in it's own way. Circuit splits end up in 3 different ways:
1. Nothing happens - different rulings stand, federal law is applied differently based on location
2. Legislative/Administrative Fix - The section of law or federal regulation, which causes the split in the first place, gets replaced with something else (usually much more specific or tolerable to plaintiffs) making the previous rulings moot.
3. Supreme Court Review - Supreme Court weights in on the issue, Supreme Court precedent overrides any inferior court decisions ending a circuit split.