Ratlines were secret escape routes that Nazi’s (or Rats in this instance) used to travel to South America at the end of World War II. Thousands of war criminals were spirited away on these covert routes.
There were two main routes — one that went from Germany to Spain to Argentina, and another that went from Germany to Rome to Genoa to South America.
There were many people involved, including (allegedly) leaders of countries, religious leaders, and wealthy fascists living outside of Germany. The physical ratlines included underground tunnels, secret airplane landing strips, and covert submarines.
New evidence has even come to light suggesting that Hitler himself faked his death and escaped Germany using this complex series of ratlines!
It was basically the evil version of the underground railway.