Labels are essentially mail folders in Gmail webmail. I think one of the forms to recover your mail (or your password) asks for four labels as part of proving your identity. You could try standard labels such as inbox, sent mail etc. but it might work better if you were able to enter custom labels (folder names) that you had created. Thunderbird once had something different that was called a label, but they were replaced by tags. Gmail webmail doesn't really have folders. They use something called a label to organize your mail that has nothing to do with Thunderbird "labels". Messages can be assigned one or more (Gmail) labels. The IMAP server maps any message with a label to a folder with the same name. See https://support.google.com/mail/answer/118708?hl=en and https://support.google.com/mail/answer/10708?hl=en&ref_topic=3394658