A extract from Eclipse help about Target Platform "Target Platform refers to the platform against which you are developing and testing your plug-ins. The target platform encompasses the location of the target, the list of plug-ins constituting the target, the target environment and launching arguments."
Next step is to import the source code for plug-ins to your workspace. This is the code with which you will be playing all the time.
Before you can start running and debusing your code, set up your debug and run configurations.