High electron density is when lots of the electrons are in one place. Because electrons have a negative charge this creates a partial negative charge. Basically a part of the Atom is slightly negative and one end is slightly positive.
With alkenes they have more electrons in the double bond meaning that it is partially negative as there are more electrons there. This means that parts of molecules that have slightly positive ends would then be attracted to the slightly negative double bond. The electrons move out from the slightly negatively charged double bond to the slightly positive part of the other molecule which is how alkenes react.
This is how I remember it. I havent done chemistry for a few months so it may not be 100% correct but its the rough idea. I would always read the textbook and then created some specific questions and take them to your teacher.