A 2/1 buy down is a loan (generally a 30 year fixed) where an extra fee is paid to "buy" the rate down for the first 2 years of the mortgage. For example, if the going market rate is 6%, with a 2/1 buy down you get 4% year 1 and 5% year 2 and 6% for the remaining 28 years of the loan. It can be good for borrowers such as Doctors right out of school, who need to ease into a larger payment and know they will be getting regular raises. But you definately need to make sure from day 1 you can handle the 6% rate in year 3 to 28. This is NOT an ARM, not negative amortization and not interest-only.
2/1 buy downs are only worth while if you are getting the SELLER to pay for the buy down. Generally a 2/1 buy down will cost 2 points. The broker origination fee would have to be on top of those 2 points for the buy down. If your seller is willing to pay for it all, why not? Or better yet you can buy the rate down for the life of the loan.