Your adjusted basis is what you paid for the property when you originally purchased it, plus what you paid for any property improvements since the date your originally purchased it.
The part that reads, "that is, your original cost or other basis of the property," takes into account that you very well could have initially acquired the property without actually buying it. (the "or other basis" part) For example, if you inherited the property then your "original cost" is the FMV of the property on the date the person you inherited it from, passed away.
If you acquired this property in 2015, then I seriously doubt (though can't rule it out of course) that it was worth less in 2020, than it was in 2015.
But overall if you have a mortgage on the property then most likely (more than 99% probable) you're going to find that come tax time, you will never show a taxable profit from residential rental property. This may not be true for commercial rental property as much.
Typically, when you account for the deductions of mortgage interest, property taxes, property insurance and then throw the depreciation in that you're required to take each year, those four items alone will commonly exceed the total rental income you get for the tax year. Add to that the other allowed rental expenses (repairs, maint, etc.) and you're practically guaranteed to operate the rental endeavor at a loss every single year. Trying to increase cost basis just so you can take more depreciation each year really doesn't help as much each year, nearly as much as it can hurt with you have to recapture it later.
