The actual quote is "Give me 50 DC-3s and the Japanese can have the Burma Road."

During the Japanese invasion of Republican China, with the coastal ports cut off, Burma became virtually the only route through which Chiang Kai-shek's government could resupply. This overland route into China, to the provisional capital of Chongqing, became known as the "Burma Road," and it was absolutely the lifeline by which the RoC clung to life.

Now, onto the Douglas DC-3. It was one of the absolute cargo-hauling workhorses of the 1930s & 40s. It could haul 6 tons of cargo, and take off from short runways (a critical detail for it to be of-use from many of the prospective island Allied airbases in WWII). If was considered one of the most reliable, universally serviceable aircraft of its era - and is still in-service to this day!

So, the essence of what Chiang was saying (albeit glibly), was that if he could get enough reliable aircraft to resupply his position, then he would no longer need to guard, fortify, and maintain the 700+ mile overland Burma Road against Japanese attacks.

This would, of course come to pass as of 1942, when the Japanese overran Burma and cut off the critical supply artery. Thereafter, until 1945, the RoC was for to be re-provisioned via air. Not primarily by the DC-3, however, but instead via C-46s taking off from British India. Their route became known as "The Hump," since it took them directly over the Himalayas. As sturdy as the DC-3 was, only the C-46s proved hardy enough to reliably brave the unpredictable conditions of The Hump. Likewise, it had a tremendous cargo capacity - more than twice that of the C-47 - making it irreplaceable as the workhorse of The Hump, lending it the nickname "The Whale." Even so, due to a variety of mechanical and electrical issues that plagued the design, the C-46 was far more infamously referred to as the "Flying Coffin" and as "Curtiss Calamity" by those who operated or rode it.

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Correll, John T. "Over the Hump to China" in Air Force Magazine no. 92.
Johnson, E.R. "The Airliner that Went to War." Aviation History Vol. 18, no. 1.
Taylor, Jay. The Generalissimo: Chiang Kai-shek and the Struggle for Modern China.