This word is somewhat obscure in modern English. In Hebrew it comes from a word that means “to boil,” so pottage essentially means soup. Genesis 25:34 makes it clear that the pottage Jacob made for his brother was a lentil soup, and verse 25:30 shows that the soup was made with red lentils.

Genesis 25:29–34
2 Kings 4:38–41
Haggai 2:12