@Alyssa Baron
I agree with bits and pieces of what's been said above, but in more simple terms, it's a way to estimate value based only upon NOI (in addition to cap rates derived from comparable deals, which are relatively easy to obtain). They are similar in nature to a gross rent multiplier in multi-family deals, which is a more straight forward metric, in my opinion.
(Note: I think there is a return relationship between cap rates and returns, however I think it's much more convoluted than using the IRR (internal rate of return), which is more closely related to the method of calculating returns for other investments (CDs, Bonds, stocks, etc).)
From an appraiser's point of view, cap rates are important because they are a major component of income capitalization approach (discounted cash flow) to value, which is one of three approaches used by professional appraisers to value commercial real estate. And unlike the sales comp or cost approach, it is the only method that uses income to derive a value, which is what investors and lenders are most concerned about anyway. In theory, all three approaches should yield very similar values, which lends legitimacy to the value estimate. (In actuality, there are many reason why they may not reconcile. So when you, as an investor, are valuing your personal income producing property or a potential income producing property deal, gathering accurate cap rate data is essential in getting the value correct (using an income approach, that is).
As a side note, professional commercial real estate investors (REITs and PE funds) and commercial lenders rely almost exclusively on DCF values, so if you're looking to get into that space, they will expect the same.
So, just to wrap up, let's say you've worked like crazy to get your pro forma income and expenses modeled correctly for a property you'd like to sell and you're confident that your NOI estimation is as close as it can be. From that point, you can gather cap rate data from comps, brokers or market publications and use that to derive a value simply by dividing your NOI by your market derived cap rate. (i.e if your NOI is $500,000 and your market derived cap rate is 8.0%, you should not sell the property for less than $6,250,000)
Apologies for such a long-winded answer, would be happy to clarify directly if it would be helpful! Good luck!