The 'wall' demand of President Trump is about how and why growing numbers of persons are trying to enter our country, especially since 2015 via Mexico and mainly of persons from Mexico and Central America.
It used to be that most of those illegally entering or residing were single men and in lesser numbers, women. Now it is more about children, spouses, whole families trying to enter. Along with than are couples once in the USA having children in the USA, so the children are automatically USA citizens and making deportation more difficult.
Many are leaving due to obscene levels of violence (particularly as to drug gangs), poverty, climate change/environmental changes (including droughts in Central America ruining farms), lack of decent paying jobs, corrupt and repressive governments, ethnic or religious repression, or on the wrong political side.
Due to treaties the USA is part of, these new migrants understand to use the 'asylum card' can at least allow them to stay and make money while their cases are pending (usually for years) in immigration court.
Employers wanting cheap, obedient, hard working and exploitable help, doing jobs most Americans won't do or just too many Americans are not good workers.
Lack of enforcement as to those here illegally and failure to set up a 'work visa' program. Lack of follow up to those violating visas to make sure they returned home after they expired. Too many who with good intentions help those here illegally including church groups like the Roman Catholic Church.
Too many who make money from helping persons get to the border and beyond.
While some extension and improvements to the wall between Mexico and the USA is sound policy, Trump seemed to make it seem to put up a 2000 or so mile wall along it. That isn't going to cost only $5 to 6 Billion, more like $20-25 Billion and still of limited use. Better and beyond the wall of Trump are to follow the recommendations DHS and political common sense. Improve patrolling and tech methods along our borders. Enforce laws as to employers or make it too expensive to hire or illegally residing persons with stiff penalties and taxes. Enlarge our immigration courts so asylum and other claims can be processed faster. More detention for asylum seekers before hearings. Figure out ways in the source countries, in particular in Central America, to make it so their people don't have to leave due to the issues that are driving them. Maybe stop the demand for the massive demand for illegal drugs in the USA that drives the gang/cartel violence causing many to leave. These are all things Democrats and Republicans must demand as what is really needed, not just a 'wall' of the imagination of Trump will be his big answer to the problem.