According to one website, "a Step-Up Certificate allows you to exercise a one-time option to reset the rate on your certificate account. If during the initial term of your certificate, the rates offered by the Credit Union increase, you can reset the rate offered."
Different credit unions have different rules about the reset process, but the main idea is that if CD yields go up during the term of the certificate, it's possible to reset the yield on the certificate to reflect some or all of the increase. And I'm happy, because although my credit union only offers three year certificates, the certificates have a really good starting yield. Most credit unions offer three and five year terms, but the yield varies from credit union to credit union