Meriwether Lewis and William Clark are two explorers that created the Lewis and Clark Expedition. Lewis was born in 1774 and he died in 1809, while Clark was born in 1770 and died in 1838.
They appear in the episode I Am Sacagawea. Strangely, they are not shown to be kids like Sacagawea. This is probably to show that Sacagawea is younger than them.