This is the text Thunderbird will use to Identify this account. The default will be your email address as shown, but you can change it to anything that makes sense to you. This name will also appear in the mail folders to identify the account.
This is the name that will be sent along with your mail as a display name, that others will see rather than just you email address appearing at their end
This address is used to complete the from part of your email. It must be a valid email address or you mail risks either not being delivered or marked as SPAM.
This email address is used to complete the replyto: header in email you send. If it is not a valid email address, you significantly increase your risk of having your mail flagged as SPAM. This is also the email address that replies to you will use, so if you want to send messages with one account and receive the replies in another, this is where you place second email address to replace the original.
This is an entirely optional field, but whatever text is included in this field is included in the message header. Unless the recipient look in the message header they are most unlikely to actually see what is in this field
Enter any simple plain text signature in the dialog. That is to be appended onto the end of your emails. Html can be used in signatures, but its use is complex enough to have a topic of it's own
ToDO link to Html Signatures
A vCard is a virtual business card. When you click the edit button you are presented with the vCard data entry window. All of the information entered in this card will accompany every email you send. So use it with care.
Outgoing Server (SMTP)
When you send a mail it uses an outgoing server, usually referred to as an SMTP server to send the mail. Each account in thunderbird has it's own SMTP server and this dialog provides access to change the server associated with the account. This is particularly handy if your home ISP does not allow you to use their SMTP server when you are not connected to their network, but to a WiFi hotspot. You can change the outgoing server to something like Gmail and still send mails from that account.
Many mail providers allow for the the use of Plus addressing, and aggregation of your email from other account into their mail server. For example Gmail offer to forward your mail to another address, while also offering to collect POP email from other addresses. Once this mail arrives in your inbox, you will want to reply to it using the correct email address, return address etc.
Identities is where this magic can be made to happen. Basically Thunderbird will look in the identities list to work out if there is an entry for the email address that the message was originally sent to. If there is a match then the identity information you have entered will be used to send the reply. It is sort of an account within an account. The field in the identify are the same as those used in your main account settings.