If something is wettable, it means that it can be made wet easily.
Wettable is a good thing for things that need to be wet, like plants or cloth.
Examples:
The soil is wettable and good for growing plants.
This fabric is wettable, so it's good for making raincoats.
The wettable sand is perfect for making sandcastles.
The paper is wettable, so be careful not to spill your drink on it!
The sponge is so wettable that it can soak up a whole glass of water.
The wettable paint makes the walls shiny and bright.