A paid-up addition is extra cash that you can put into your policy that goes to increase cash value and death benefit. The insurance company sends you a letter annually stating the minimum ($250) and the maximum PUA. It’s your choice every year how much you contribute.
On most policies, the minimum PUA is $250, the maximum is whatever is specified by the terms of the policy, usually about equal to the base premium. If your base premium is $10k/year, your maximum PUA will be about $10k a year, or a total of approximately $20k per year for “mid-age” folks. This is affected by age. (But do pay attention – the allowable paid-up additions will fluctuate as the death benefit rises.)
Since it is the Paid-Up Additions that really super-charge the accounts, remember to aim to fund the maximum contribution rather than the minimum base premiums. Every dollar above the premium becomes a PUA dollar, and it is very advantageous to overfund your policy up to the MEC limit.