Scout wears a ham costume in the pageant. It was made for her by Mrs. Crenshaw, a seamstress, who shaped chicken wire into the form of a cured ham, covered it with brown cloth, and then painted it to look like the real thing. Jem told Scout she looked like "a ham with legs" in her costume. The costume restricted Scout's movements and interfered with her vision, but it also saved her life when Bob Ewell attacked her (and Jem) while they were walking home after the school program.
Scout is in a pageant about Maycomb County's agricultural products, and she wears a costume that resembles a ham. The costume was made originally of chicken wire covered in cloth, and it came in handy as she and Jem walked home from the evening's pageants -- they get attacked by Bob Ewell, and Jem gets his arm broken by the villain, but Scout is protected by the confines of the costume and remains safe.
There is a double entendre in the fact that Scout is dressed as a "ham:" she is both portraying a literal ham in the pageant, but figuratively, she is acting, meaning that she has to also be a "ham" in that regard as well.
Scout is a ham and this costume is what actually saves her. This costume is fat and although heavy, when Scout gets attacked, the costume takes most of the hit and saves her from being killed. This costume also limited her vision and she was going to take it off, but since they had a bad feeling about what may happen they simply kept on walking.
scout is a ham made out of chicken wire- that saves her life :)