Yes! It is true. There is a law already in existence called the Secure Fence Act. The law was passed in 2006 with then Senators Hillary Clinton and Barack Obama voting “yes” for the construction of a security fence along the border with Mexico. The law was passed with the support of both Democrats and Republicans. It authorized the Secretary of Homeland Security to “take all actions the secretary determines necessary and appropriate to achieve and maintain operational control over the entire international land and maritime borders of the United States.”
According to Byron York of the Washington Examiner, “the law called for the usual mix of high-tech sensors, cameras, border checkpoints, vehicle barriers and other measures along the U.S.-Mexico border. And then it ordered Homeland Security to “provide for [at]* least 2 layers of reinforced fencing and the installation of additional physical barriers for hundreds of miles on the southern border.”
This means that President Trump does NOT need Congress to act nor does he need an executive order to have a security fence constructed. He only needs to have the Secretary of Homeland Security John Kelly to implement the law as specified in 2006.