You can use this page to set your personal information. At the top, if enabled, are options for selecting the current identity, creating new ones and deleting them. The name, email address and reply address are all part of an identity. If identities are not enabled, you will only be able to use a singleset of values. Also, if you or you administrator has disabled the changing of your email address, you will not be able to set this, only your reply address.
The other options are your language and whether to use this choice instead of that at logon. This is useful if you don't wish to use the default choice and don't wish to always have to change it. Your signature is used when composing mail, and the checkbox underneath determines if it's used for replies and forwards.
If any of the options here are changed (except for adding/deleting identities) you will need to click the "Save Changes" button to commit them.