Egusi is melon seed (a different specie from watermelon and the seeds look very much like pumpkin seeds). The seeds are dried and shelled, shaft is separated and then it's ground into a powder/paste.
The powder/paste is cooked in palm oil with tomatoes and pepper. Any kind of meat/fish can be used.
Egusi soup is a staple both amongst the Ibos and Yorubas in Nigeria.