If a liquid seethes, it is boiling, but if it's a person who's seething, watch out! He or she is really angry!
In the cooking sense, to seethe means to boil and if you are seething with anger, you might say that your blood is boiling. However seethe can also mean filled with activity. Imagine a room seething with excited people. Now imagine that you are watching that room from a balcony. See how the people move here and there? It's almost as if they are bubbling around.