There are several million illegal immigrants in the U.S. who were brought into the country as children. About 750,000 of them gained a fragile legal status under Obama. They're known as DREAMers.
The first DREAM Act (Development, Relief, and Education for Alien Minors) was introduced with bipartisan backing in 2001. It offered temporary legal status for people brought to the U.S. before 16 who could pass a background check and had a high school diploma or GED. Permanent legal status could be attained through college attendance or military service. The legislation never got through Congress.
In 2012, President Obama announced he would stop deporting those who met the criteria of the DREAM Act. Two-year renewable permits were issued granting DREAMers work authorizations and protecting them from deportation.
Trump has vowed to reverse all of Obama's executive actions. This would be on the list. But about Dreamers, he told TIME Magazine he wants to "work something out that's going to make people happy and proud."