In Florida, and I suspect a lot of other places that allow a mail in ballot, the signature on the envelope in which you submit the ballot must match the signature on file with the Supervisor of Elections. If its been a awhile since you registered, remember to update your registration signature so that your ballot does not get rejected. Signatures change (mine has changed a lot).
In Florida, you can download a new registration form, mark it as a record update, complete it and mail it in. Don’t let your ballot get rejected.