Scout is costumed as a ham in this scene, and the costume is made simply from papier-mache and rustic chicken wire. The hole through which she is forced to peer is small enough to limit her field of vision, and makes everything around her seem far more drastic than it may actually be.
Her senses of hearing and touch are heightened by the costume's limitations, and Jem is engaged as her guide down the path toward home.
Eventually, Scout is attacked by Mr. Ewell, who is angry with her father for defending Tom Robinson, among other things. Evidence from the costume later indicates that Mr. Ewell attempted to stab or slice at Scout a number of different times. In the end, it may be that her "ham" costume was what saved her during the attack in the first place.
Because Scout is "trapped inside" a costume made of chicken wire and paper maché, she can't see or hear what's going on outside. This magnifies her anticipation of "something out there" even if for the moment she's sensorially deprived.
Scout can only count on Jem and feeling her way along the path to know what's going on. Ironically, when Mr Ewell finally attacks her, it is the constume which saves her until Boo intervenes. The slash marks on the chicken wire prove that Mr Ewell had indeed made more than one stab at her. Jem escapes with a broken arm which never really heals properly, but Scout is evidently the victim Mr Ewell was primarily after.
This affects her understanding because she could not see or hear as well as Jem could. The costume she wore made her be off balance as well so when the attacker (bob ewell) was chasing them and Jem and Scout tried to run she lost her balance and rolled away not able to see what was going on, but she heard noises and jem screamed (thats when he broke his arm) so she couldnt really understand what was going on