Before AFS can know who you are, you have to be authenticated to the cell. Authentication is just the name given to confirming your identity. There are several ways to authenticate to AFS. The most common way will be through the attach command. If you attach a filesystem that is in AFS, you will be authenticated automatically to it. Should you have to reauthenticate for some reason, you can just reattach whatever it is that you are trying to use.
attach authenticates to AFS by using a program called aklog. Generally, typing aklog with no arguments will reauthenticate you to whatever you are using. To be safe, however, you can specify the path of the file you need to reach. For example, if you are working in the potato locker, you can type aklog /mit/potato to reauthenticate.