Secretly eating something (often without permission), especially sneaking bites in the kitchen or elsewhere.
母は私が夜に台所で盗み食いしているのを見つけた。
My mother caught me sneaking food in the kitchen at night.
子どもはお菓子の盗み食いを親に叱られた。
The child was scolded by his parents for sneaking sweets.
夜遅く、冷蔵庫の残り物をこっそり盗み食いした。
Late at night I secretly ate the leftovers from the fridge.
Essentially the same: taking and eating someone else's food or food you shouldn't eat, done stealthily.
冷蔵庫のケーキを盗み食いした犯人は弟だった。
It was my younger brother who sneaked and ate the cake in the fridge.
試合の日、チームの差し入れを盗み食いしないでください。
Please don't sneak and eat the team's refreshments on match day.