She was a young woman who had been captured by the Hidatsa tribe and later married to a French fur trader named Toussaint Charbonneau. Lewis and Clark hired Charbonneau as their interpreter, and Sacagawea came along as his wife.
While Sacagawea wasn't fluent in English, she spoke several Native American languages, including Shoshone, and was able to act as a translator and guide for the expedition. She was also instrumental in helping them find the Shoshone tribe, which proved crucial for their success in crossing the Rocky Mountains.