But if the solution is saturated, then no more will dissolve. So if you add some more of the substance, it will not dissolve no matter how long you wait or how much you stir it. Polar things dissolve in other polar things, but not in non-polar things. So oil or grease or any non-polar substance will not dissolve in water. Therefore, water alone won't remove much oil or grease from clothes.
Soap is a complicated molecule.
It is a long non-polar hydrocarbon, but on the end is an ionic group such as carbonate. So a soap molecule has a non-polar part and an ion on the end. The long non-polar part can dissolve in a droplet of oil, but the ionic end will stay outside the drop where it can attract water molecules. The droplets of oil can then be moved by the attraction of the ionic part to water. The oil does not truly dissolve, but tiny droplets are suspended in water and washed out that way.