According to the New Yorker, who quote "several sources",
"Kelly explained that he had spent a great deal of time talking through the issue with Trump, and he believed he had convinced the President that he didn’t actually need to build a physical wall along the entire nineteen-hundred-mile-long border between the United States and Mexico. Instead, the use of sophisticated monitoring technology, air surveillance, and fencing could secure the border with what Trump could start calling a “barrier.”
Just putting the issue in a more reasonable context.