I see ads in the paper to sign up for otzer beit din and get a prepaid card. What do I need to know and is this important?
Ozar Beis Din is a way in which Shemittah produce can be bought without transgressing the Shemittah prohibitions against marketing Shemittah produce.
The payment is to cover expenses of the workers etc. and not for profit purposes, which is why it is permitted.
There is nothing special that you need to know, and with the prepaid card you can simply purchase the Shemittah produce from the relevant outlets without concern for the Shemittah prohibition. The produce of course will be sanctified with the Shemittah sanctification, and must be treated with due respect.