Short answers, pulled from the story.
The Romanian Bridgehead was an area in the southeastern corner of the Second Polish Republic, now located in Ukraine, that Polish commanders designated as a fallback defensive zone during the German invasion of Poland in 1939. The plan called for Polish forces to withdraw to the hills along the borders with Romania and the Soviet Union, where terrain and pre-positioned supplies could support a prolonged defense.
Marshal of Poland Edward Rydz-Smigly, the Polish commander-in-chief, ordered approximately twenty divisions to withdraw toward the Romanian Bridgehead on the 14th of September 1939. After the Soviet invasion on the 17th of September, he issued a follow-on order directing all remaining units to withdraw to Romania and Hungary.
Poland and Romania had been allied since 1921, but Poland chose not to activate the defensive pact because keeping Romania neutral preserved access to the port of Constanta. Polish planners judged that an open corridor for Allied resupply through Constanta was more strategically valuable than activating the alliance and drawing Romania into the war.
As many as 120,000 Polish troops withdrew through the Romanian Bridgehead area into neutral Romania and Hungary. Most of those soldiers subsequently joined the Polish Armed Forces in the West, serving in France and the United Kingdom during 1939 and 1940.
In 1939 the Romanian government received the treasury of the National Bank of Poland. A portion consisting of 1,261 crates containing 82,403 kilograms of gold was loaded onto a commercial ship at the port of Constanta and escorted by Romanian Navy ships to Western Europe. A second portion was deposited in the National Bank of Romania and returned to Poland on the 17th of September 1947.
The plan depended on the French launching a significant offensive on the Western Front, which never materialized. The decisive blow came on the 17th of September 1939, when the Soviet Union invaded Poland from the east under the terms of the secret protocol of the Molotov-Ribbentrop Pact, creating a two-front war that fragmented Polish units and closed off the escape corridors before most forces could reach the bridgehead.