Wash wash powder is the detergent you put in the wash you make sure your clothes are washed well. Detergent used to only be available in powder form back in the old days, and it still can be, so this is for that good powdery stuff.

This is a mix of the on'yomi readings for 洗濯 and the kun'yomi reading for 粉. You already learned this reading for 粉 when you learned the word 麦粉. If you can remember that, you should know the readings for this one too!