"My broker is offering a biweekly mortgage. What does this mean, and how does it affect my payments?"
When you make payments every two weeks, rather than once per month you will actually make 13 payments over the course of a year.
The bi-weekly option actually pays more towards the principal and interest per year because there are more payments made in the course of one year. This is a very nice option for people that want to accrue more equity in their home just a little quicker. And can be deducted from you checking account on regular intervals, therefore making it easier for some people to budget it on a check to check basis, instead of a monthly basis.
You can pay your mortgage bi-weekly if you want to. There is no requirement that says that you have to sign up with any service to make this happen.