This is a document which states that you are free to marry. Some countries will require that you prove your marital status before accepting your application for a marriage licence.

To obtain this document in the UK you should go to the Register Office at your local Town Hall - a Notarised Declaration ("Affidavit") signed by two persons before the Notary public, declaring that they have known the applicant for more than two years and that to their knowledge there is no impediment to his/her marriage. Please note that to be valid in Brazil this document should be legalised by the Brazilian Consulate in the UK and translated by a Sworn Translator.