In some countries, translations of documents required for legal or official purposes must be sworn to confirm that the translation is the legal equivalent of the source text. This can only be done by sworn translators.
Depending on the country, a sworn translator swears an oath before a high-ranking court official or is required to pass an examination to designate themselves as a sworn translator.
There is no such thing as a sworn translator in the UK. The equivalent, accepted by courts and public authorities, is the certified translation.