President Donald Trump’s executive order denying U.S. citizenship to the children of parents living in the country illegally has faced the first of what will be many legal tests. It didn’t fare well.

A federal judge temporarily blocked President Trump’s order seeking to end birthright citizenship for the children of many migrants, a major hit to one of the president’s Day 1 orders.

Here's a guide to what birthright citizenship is, what happens if Trump's order ends it, and what comes next after lawsuits were filed.