Bitter herbs are some kind of bitter-tasting vegetables, perhaps a certain variety of lettuce. The people of Israel were commanded to eat bitter herbs along with roasted lamb and unleavened bread on the night when God caused the plague of death on all the Egyptian firstborn (Exodus 12:8–11).
The "bitter herbs" mentioned in Exodus 12:8 and Numbers 9:11 likely refer to plants such as endive (Cichorium endivia), the common chicory (Cichorium intybus), lettuce (Lactuca sativa), or the common dandelion (Taraxacum officinale). All of these are weedy plants commonly found in modern Egypt and western Asia. People living in these regions still eat these plants today.
The leaves of ordinary garden lettuce become very bitter when bleached (when they turn white after being covered to prevent sunlight from reaching them). This is also true of the common dandelion. Some scholars suggest that the bitter herbs might have come from thorns and thistles.