What Is Periodic Pension Cost?

My pressing concern is with the total periodic pension cost. To be frank, I do not understand what this is getting at. My notes say it is the following:

Ending funding status less employer contributions less beginning funding status.

I do not see how this means anything, much less the cost to the firm. Let me run a simple example. We start underfunded by 10k; we end overfunded by 10k; and we contribute 5k. That equals 15k (10 – 5 – -10). How did this cost the firm 15k during the period? I can find no clear explanation. If anyone would like explain this and/or point me to a suitable video, that would be appreciated.