It is all in the numbers. Real Estate can behave like a bond once it is rented long term. Look at it this way. You invest in a property, which is like buying a bond, and the monthly rental check is similar to a bond coupon. Now the ratio of the price you paid for the property and the rent you receive annually is equal to your yield. If the property cost you $200,000 and you receive a total of $12,000 in rent you would be yielding a 6.00% return.
If you could borrow the entire purchase amount at an interest rate lower than your yield you would have money in your pocket at the end of the year. The property would yield a positive cash flow. Likewise if the mortgage rate was higher than the rental yield you would have to chuck in some of your own money to pay the mortgage.
It is important to understand that in this example we assumed an interest only mortgage. In real life the mortgage lender would most likely require a repayment of the principal over 15 or 30 years. As repayment is like savings it changes your cash flow but does not alter the yield.