What is a ...

Hi: do you mean the pin that goes into the router plate when it is table mounted?
If you do that is called a starter or safety pin. Its used toallow you to get material to the router bit without kick back. If you were going touse a bearing guided bit
around a template, it allows you to get the cutting started, and then you can pull
away from the pin.. Hope this helps.. Woodnut65

You may be talking about a setup pin used to make sure that your router is mounted perfectly to your baseplate. If you use template guides, and the router is just a little off of center, than the distance between the template guide and the cutting edge of the bit will vary, leaving an wavy cut if the router is rotated any as you are cutting.

If you chuck up a centering pin, or bit, in your router, then put a small template guide in, you can then check that you have uniform clearance and the router is centered.

(If you don't work with template guides, you don't need to worry about using one. If your cheap - like I tend to be - chuck up the biggest straight cutter you have, find a brass template guide has the same diameter, and make sure the bit turns freely inside)

Hope this helps,