The law requires annual contributions to qualified pension plans to ensure they remain adequately funded. You can either contribute annually based on the formula for plan assets/plan liabilities or, you can do what Truist does and "prepay" the minimum contributions.
If you take a look at the statement, the plan actually has ~12.5bn in assets and 6.5bn in liabilities. However, when you calculate the minimum funding amount you subtract from total assets what assets have been contributed for the purpose of "prepaying" the plan. So, annually, instead of making additional contributions, you just move the amount of minimum funding required from the prepaid component to the plan asset component. Now, if our plan liabilities remain relatively consistent (they should), then to burn through the "prepaid" component of the assets at 300m a year will take 15 years. (5bn/300m)
This is a rough explanation as there are complexities to the actuarial calculations (death rates, interest rates, etc.)