When you set the "Default Context" field of a project, you're effectively saying "most of the actions from this project will be in this Context". When you create a new action, it'll start out in that context, though you can edit the action to put it in another one whenever you want.
This extends to action groups, as well; by default, child rows inherit the context that their parent has.
Re "waiting-for" actions - some folks use on-hold contexts; other folks defer them by way of the start date field. Both approaches are summarized well in the early posts of
this thread, if it helps. (It also touches on when you might want to set a whole project to "On Hold".)
That'll probably get you started - feel free to post any follow-up questions you may have and we'll be happy to help!