The way the pixels from one layer in a montage interact with those in the layer below. In ‘Normal’ blend mode, the pixels on the top layer completely cover the ones below, in ‘Multiply’ mode their values combine to produce images that are darker overall, in ‘Overlay’ mode, pixels lighter than 50% grey lighten those in the layer below while pixels darker than 50% grey darken them… there are many other blend modes besides these.