Typically a P&L is also called an "income statement."
The "Net Operating Income" is just that -- income from operations. It usually doesn't include "1-time" or "special" or "non-operations" part of your business.
At the end of the day, interest expense is a tax-deductible expense that is the cost of your "capital" structure. You get to choose your capital structure, so usually we don't include that as an "operating" expense and it is not included in NOI. So you "add that back in."
Depreciation is as @Randy_CA said, a real cost but it's not a cash expense. Since it's not "cash out the door" you typically "add that back in" when trying to figure out what a property is going to actually cash-flow.
This is what people mean when they say, "add back" the depreciation (and amortization) and interest expense.
You would add back any "special one-time big expenses" too if you knew what they were, because that should not be included in your "Operating" income as "special big one-time expenses" are usually investments in your capital operation and not an operating expense.
As a quick-and-dirty, you can look at the P&L, add back depreciation and interest, and that's about the amount the property will "cash-flow" before debt service. For your cash flow after debt service, you take the NOI and subtract the cost of your financing (usually debt service interest AND principal!) minus any of those one-time expenses that are not "ordinary." But you still have to plan for those!
I wholeheartedly agree take a simple basic accounting course.
What you get back at tax time is a separate issue. Assuming you buy the park and you do well, you'll have some cash flow, and some net income, which is a different number from NOI, and you'll owe some tax on that net income.