I have a 'twin needle' in the stuff that came with my machine. Can someone tell me what it's for?

My mom always used a twin needle for two rows of topstitching, especially on t-shirts.

I found this note on a website, when searching for the use of twin needles.

Note: Twin needles are ONLY for machines with the bobbin located in the front.

Does this mean the bobbin is inserted from the front rather than the top? My machine came with a twin needle, but the bobbin in loaded on top.

I would think it means from the front, not the top. I have never heard that and why would the machine come with it if you can't use it? Have you looked in the sewing machine book?
What kind of machine do you have?

I have a Brother CS6000i. I'm not sure why it came with one, based on that note.

I googled and I didn't see that. Two different sites said that most machines can use a twin needle if they have zig-zag capabilities and a needle plate with a horizontal hole to avoid needle breakage. So if you have a zig zag foot I think you can use the needle.

I looked up your machine and it says right on Brother's website that an attribute to the machine is the twin needle.

I've often used twin needles in my top loading bobbin machine..no problems. I've used them to do 2 rows of topstiching, and you can also so pin tucks but you have to play with the tensions quite a bit, both top and bottom tensions (on my machine anyway).

I have a top loading bobbin on my Janome and have used twin needles many times. You have to make sure that your stitch width accomodates the size of needles you have. If your machine only has a 5mm width you can't use a twin needle that needs 7mm.

If you use a twin needle to do topstitching, be aware that the stitching on the underside is very different than what you might expect; it form a zigzag between the two rows on top. This makes it nice for topstitching knits, because the zigzag underneath gives the stitch some stretch. The bobbin tension usually needs to be loosened for this type of stitch; otherwise the fabric creates a "tunnel" between the two lines of stitching (from the zigzagging being too tight).