Insoluble means a material that can not dissolve in a liquid. For example if you put flour in a cup of water it will not dissolve you can tell that it is insoluble because the material that has been put in a cup of water hasn't disappeared.
What Does Insoluble Mean?
Insoluble substance is a substance that cannot be dissolved. An example would be calcium in blood, blood is primarily water and water cannot dissolve Calcium.