Geography affects American border security by making some borders easier to cross and/or more likely spots for illegal crossings while making other borders relatively difficult to penetrate.
For the United States, the East and West Coasts are relatively difficult to penetrate due to their geography. They face onto large oceans and are mostly quite far from any foreign countries. By contrast, the northern and southern borders are very long land borders. They go through very remote areas and can be very hard to patrol. The northern border, though, is typically seen as a less likely spot for illegal crossings because it borders on a rich country as opposed to a relatively poor one. In these ways, geography determines which US borders are most likely to be crossed illegally.