Money market mutual fund

I am 24 years old and starting to get a handle on my financial situation. I have started an emergency savings account and am ready to open a Roth IRA with $2,500. I started an application for an account, but it asked for my choice of a sweep fund. Nothing I've read has mentioned this and my Internet research has come up with limited and confusing information. What is a sweep fund and how do I choose one? Julia, Boise, ID