Similar to Photoshop Layers, or Unity Terrain layering system it will the weight of each input taking into account a priority system to ensure that the sum of all outputs is 1. This means that the lowest on the array, the higher the priority.

Notice how the first Weight Output is the input at 0 without the Input 1

This node is useful in collaboration with Asset Outputs to ensure that the textures have a clear ordering when being selected for the terrain.