Breakdown of Les chaussures dans la vitrine sont en promotion.
Questions & Answers about Les chaussures dans la vitrine sont en promotion.
Les means the, while des means some.
In Les chaussures dans la vitrine, the speaker is referring to a specific set of shoes: the shoes in the display window. The phrase dans la vitrine identifies which shoes we mean, so French naturally uses les.
If you said Des chaussures dans la vitrine sont en promotion, it would sound more like Some shoes in the window are on sale, which is possible, but less definite.