Contexts are used to help you choose a task that is suitable for where you are and what you are doing. What context you are in, in fact. Here is a suggested list of contexts to get you started.
You can give all your tasks a context, then filter on all the tasks to be done at work, for example. Divide and conquer! Then you can stop stressing about tasks you can’t do at the moment.
Now pick a task you can do, and just do it!