This simply means that you have all your helpful spells bound to various mouse keys with shift/ctrl/alt modifiers. All healing spells should be cast on mouseover while damaging spells cast on the active target. This can be achieved through the use of mouseovers which cast a spell on the target below your cursor without the need to switch active target. It is strongly recommended that your UI is set up in such a way that means you never need to switch off of having an enemy targeted.
This style of play can be very inefficient and cumbersome to achieve. As a result, newer players quite often switch their active target from enemies to friendlies continuously. Discipline spends a lot of time dealing damage to enemies for Atonement healing. While they are not mandatory, addons, macros and a well set up UI can make your time as a healer both easier and more efficient. Due to personal preference, the end look will always vary from person to person. There are many different ways of setting up your user interface.
