A federal judge Thursday blocked President Donald Trump’s executive order outlawing birthright citizenship —the policy automatically giving citizenship to anyone born in the U.S.—and declared it “blatantly unconstitutional,” as legal experts widely believe the president does not have the power to change the policy on his own.

Trump's order directs federal agencies to stop issuing citizenship documents to U.S.-born children of undocumented mothers. It also applies to mothers in the country on temporary visas, if the father is not a U.S. citizen or permanent resident.

One of the many executive orders President Donald Trump signed on Monday included one to end birthright citizenship. Here's what to know about it and why lawsuits have already been filed.