Not everyone is able to hit the qualifying standards for Boston, especially as they continue to get more competitive year after year. As a result, the race allows some runners to enter via fundraising with one of the preapproved charities.
In order to race for a charity, you will need to apply separately with that charity. Each charity has their own application and selection process. If you are selected, the BAA requires a fundraising minimum of $5,000, and each charity may have different requirements as well. You should apply with a charity in September of the year before you'd like to run the marathon.