Why did people take iodine pills after Chernobyl exploded?

In the HBO miniseries " Chernobyl ," the Soviet nuclear physicist Ulana Khomyuk (a composite character played by Emily Watson) realizes that there's been a massive release of radioactive material somewhere nearby and immediately pops an iodine pill .

from FOX News https://fxn.ws/2JYsqd9
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