If you are using Firefox, Netscape, or Internet Explorer, download the certificateand follow the prompts to install the certificate. This certificate is issued by the the University of Michigan and used to authenticate UM internal web sites, such as phorum.eecs.umich.edu.
For Safari, click on the above link as well. But Safari will not automatically install the certificate. Go to the Safari menu -> Window -> Downloads. You should see that certificate file has been downloaded. Double-click on the downloaded file to install it.