I certainly can't claim ownership of the "next action" concept, as I first heard about it when reading David Allen's "Getting Things Done" several years ago. However, the actionable next step is something that I find 100% crucial to making progress any of my goals, tasks or projects.
The more vague the task, the more amorphous it seems. If you can identify one thing, no matter how small, that you need to do next, you will make progress. Defining an actionable next step for each and every task and project allows you move forward.
Sometimes, the next step is simply "schedule time on calendar to brainstorm ideas for X" or "call X to ask about Y". It is very hard to procrastinate on something that is clearly defined because you will find you just can't muster the excuses. It is very easy to procrastinate on something vague like "plan launch event", because it feels big and you aren't sure where to start and so you think, "I'll just start this one tomorrow".
Taken a step further, if you can take a minute to define what the several next steps are for a task or project, you will be able to execute efficiently. As soon as you've added a task or project to your task list, take 30 seconds to define the next step, and what the next steps after that are likely to be. Knowing what you path is allows you to follow it without stressing about what's next.