A certification (cert) is a soldier attribute that allows him or her to operate specific classes equipment.
Some certifications have prerequisites that must be obtained in order.
Certs can be forgotten every 6 hours.
New Characters have a 7 day grace period, and don't have a penalty for unlearning Certifications. After these 7 days, they will have to wait 6 hours to unlearn Certifications again though.
You may only forget one certification at a time. However, if you forget a cert that serves as a prerequisite for others, any dependant certifications will be forgotten as well.
For example, if you have a BFR certification and you forget Armored Assault I, you will lose three certs: Armored Assault I, Armored Assault II, and BFR. If you also had BFR Anti-Aircraft, BFR Anti-Infantry, and Flail, you will forget those as well.
With the release of version 3.11, characters will receive the option to "forget all" certifications. This option is only usable once, until the next major release, at which time each character will again receive the option.