Your priorities muscle is strong when you are able to identify the most important things to focus your attention on when facing adversity. This muscle is useful because it helps you avoid two traps: trying to take care of too many things at once, and spending energy on things that are not critical.
Some simple daily exercises you can use to strengthen this muscle include:
Saying No: Each day, identify one thing that you can say no to because it is less important than the other things you are doing.
Visible Priorities: Each morning, take a few minutes to identify your highest priorities for the day. Keep them in mind as you go through the day, and at the end of the day consider how well you stayed focused on them.