WASHINGTON — Standing in the way of a resolution to the government shutdown is a wall. A still mostly hypothetical wall along large sections of the southern border intended to keep immigrants from entering the United States illegally. A wall that President Trump promised his supporters he would deliver once he was in the Oval Office. A wall that if not built, some conservatives say, could cost Mr. Trump his presidency.

It is a big wall. Here’s what you need to know about it.

Building a wall along the southern border was one of Mr. Trump’s most repeated campaign promises.

As he described it during the campaign, the wall would not be one continuous barrier along the entire stretch of more than 1,900 miles of land between the United States and Mexico, but rather sections of a wall. Natural barriers and “Border Patrol people” stationed in certain parts of the Southwest would provide security in between.

And building the wall would be easy. “I’m a builder,” Mr. Trump said. The construction would not be as complicated as building a 95-story high-rise. “And I would have Mexico pay for it,” he said. “Believe me.”

The estimated cost of the wall would be $4 billion to $8 billion. It would be 35 feet to 45 feet high, with a “big, fat beautiful door right in the middle,” he said, which would be for people to enter the country legally.