There’s detergent under the shelf in the washing area.
You already learned that the word 洗浄 means "washing." So when you're washing the laundry you have to use just the right dose of detergent to make sure they wash properly. If you use too little detergent they'll still smell, but if you use too much detergent the washer will explode with bubbles! Just be careful okay, don't end up like me.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.