While importance and urgency might appear similar at first glance, they are not the same thing. Two examples might help you understand the differences between them.
Let's say it's January and your company has decided to move its headquarters to a new location by the end of the year. This would be an important activity for the company, but it is not very urgent. Selecting a new site can make or break some companies, but having a year to do it should be sufficient time. Therefore, you would assign this activity a priority of A5-a high importance but a low urgency.
On the other hand, let's say it's 11:30 AM, and you decide you want to go to lunch with a co-worker. You want to send them a question activity that will get their attention. This is an activity of some urgency, but it is not very important overall. Therefore, you would assign this activity a priority of C1-a low importance but a high urgency.
While these two examples are extreme, they can give you an idea of the two considerations you should make when fixing a priority to an activity.
If you can't decide on the importance and priority of an activity, you can place the activity on hold (by using urgency of 9) until you do decide, or you can accept the default priority of B3.