Because immigrants were crossing the Rio Grande river in Texas, Republicans put out barbed wire to be evil.

Anti immigration and racism are big issues here. There is a popular idea that illegal immigrants swim across the Rio Grande river to cross the border, so as a publicity stunt he had razor wire put up so that anyone attempting to cross would either drown or bleed to death.

Razor wire is like barbed wire but with literal razor blades instead of pointy bits. It is designed so that attempting to climb over/through it will shred your flesh such that you get tangled and bleed to death before being able to get out. Horrific stuff, usually used to protect airports, prisons, etc.

The rolling hypocrite was treating the border areas as literal war zones, and (ordered his lackeys to) place razor wire on the banks and sometimes in the Rio Grande.