The IRS, through a partnership with several private companies, now allows you to file your taxes on-line. If you want to use the web to file, be sure to use an IRS authorized site. I want to publish a website, but I only want certain people to be able to see it. Certificates are electronic documents included in your browser. When you connect to a secure site certificates allow encryption and authentication to happen.
Most browsers have certificates from several of the larger CAs already installed.
For most of the secure websites you encounter you will not need to install a certificate. If you do encounter a site that uses a certificate not already installed in your browser, a dialog box will pop up notifying you that to make a secure connection you will need to install a certificate. Read this dialog box carefully. Check with the organization running the site to be sure you have the correct URL and that the site is a trustworthy site. People on the same network as you could intercept and steal your unprotected password using different sniffer programs.