Explicit enhancements are predefined enhancement sections by SAP. They are stored in Enhancement Spots. They can be implemented by going in the ABAP editor, get into the Enhancement Mode by clicking the spiral button, then, right click into the enhancement point you plan to implement and select Enhancement Implementation->Create. You will be asked a name and a description and after this you can simply add your code.
Implicit enhancements are predefined at the beginning and end of includes, FM's etc.
You can see what are available by going into editor and go to edit -> Enhancement Operations -> Show Implicit.....
Thanks
Naidu