Hydrophobic is term to describe something that does not mix with water. Hydrophilic is the term to describe something that likes water. If this is to describe an ingredient in sunscreen, this means that it will not dissolve in water. Hydrophobic things are oily such as hydrocarbons, fats and oils. Hydrophilic things will easily dissolve in water such as salts like phosphates, sulfates etc. They hydrophobic chemical is probably used in waterproof sunscreen. This means the sunscreen will stick more to the oil on your skin, and won't wash off in water. It will still wear off your skin over time just by rubbing of clothing etc, but there is no chemical reaction taking place.Soap would also be able to wash off the hydrophobic ingredient.

Hydrophobic just means 'afraid of water'.So something that is hydrophobicrepels water. Oil is hydrophobic, therefore oils or Vaseline are put in lotions or sunscreen to repel water (such as sweat and pool/ocean water). Silicon is also hydrophobic so it is sometimes added too. When the lotion wears off (with lotions it is basically wiped or rubbed off) their hydrophobic properties are worn of as well.

Van Der Wall's forces - the electrons are constantly changing position in any molecule, and polar molecules that have electric dipoles like to line up like magnets (and for exactly the same reason as magnets). Polar molecules will bond to other polar molecules, non polar to other non polar, and you'll get separation.