The border is the international boundary between America in the north and Mexico in the south.
The Mexico-United States border was the most crossed, controlled international border in the world, with some 350 million (legal) crossing every year. It spans a length of 1,989 miles and crosses everything from uninhabited dessert to major metropolitan areas.
The border seems to have been a hotbed of activity during the outbreak. According to George Geary, elements of the Mexican Military were deployed to secure the border and to stop the infected (and living refugees) from crossing into Mexico from America. At the time Madison Clark crosses into Mexican waters aboard The Abigail, only elements of the Mexican Navy were operational in Baja California.
According to Sierra, the border was a nexus of activity, with people fleeing both south and north in order to escape the walker epidemic. Sierra's traveling companion Gabi claims to know a safe haven in Mexico, which makes the border the destination for Gabi and Sierra. Gabi's boyfriend Colton knows about a tunnel under the border, which is the intended destination for Gabi and Sierra.
The border is also where Nick and Luciana lead the remaining La Colonia survivors, in hopes of finding a refugee camp, as they witnessed a helicopter take off from what appears to be a military encampment on the border. They head towards the border crossing and find it abandoned, but the helicopter is seen again - far off in the distance. Unfortunately, they were attacked by several members of the Broke Jaw Ranch militia, led by Troy Otto.