https://www.investopedia.com/terms/p/presentvalue.asp
Present Value=Future Value / (1+r)^n
n = number of years in the future the liability is due
r = rate of return (also termed discount rate)
For engineering life cycle cost analysis I was doing, we used r as set by the Ontario Ministry of Finance. MLB likely uses the value set by US Department of the Treasury. r changes with economic conditions, so Ohtani's impact on the luxury tax is going to vary each year of the contract.
My experience with it is in engineering. As an engineering example, you might want to compare the alternatives of rehabilitating a structure vs. removing and replacing with new. The two alternatives will have different costs over the analysis period (for a structure, 100 years is typically used - beyond 100 years the present value of these costs will be comparatively negligible anyway). The future liabilities that apply to each alternative are converted to present value for the purpose of cost comparison.
A made up example at a basic level:
Rehabilitation - $80M now, next rehabilitation in 25 years $100M discounted to present value, remove and replace in 60 years $250M discounted to present value
Remove and Replace - $250M now, 1st rehabilitation in 40 years $50M discounted to present value, 2nd rehabilitation in 70 years etc, etc
A more involved analysis will include operating costs and any revenues for each year of the 100 years, which will vary between the existing and new structure.
The Jays likely did such an exercise before deciding to renovate the Rogers Centre rather than construct a new facility.
Applied to Ohtani's case, it is a method to estimate the true Dodgers payroll cost. Obviously the true cost is not counting future payments as if they are paid today. A dollar 20 years from now does not have the same purchasing power of a dollar today.