Aqueous Solutions

Most Ionic Compounds will dissolve in water, and when they dissolve they disassociate into their respective cations and anions. The number of Ions that are in a Ionic Bind dictates how many Ions are disassociated, for example NaCl disassociates into two Ions but MgCl2 disassociates into three ions.
So before I start to explain a unit about something, I should probably go over the general thing, which in this case is Aqueous Solutions. An Aqueous Solution is a mixture that takes place in water, with the solute being dissolved into water, which is the solvent. Examples include Chlorine dissolving in a pool.

However Polar Substances will disassociate based on their attraction to water, while most insoluble materials are non-polar.

If a molecule contains -OH then the shorter the C-H chain is the more likely it is to dissolve in water 
Crash Course helped me understand this better HERE
Oh,and understanding polarity might help, so HERE
And a visual of Polar vs Nonpolar: