A certified translation is a type of translation required by some organizations such as the Home Office, insurance companies and educational institutions. This type of translation is carried out by a certified translator. There are different levels of certification. The most basic version is sufficient in most cases, and consists of a translation with a declaration and stamp to formally verify the translation for official use.
The level of certification you need depends on the requirements of the organization you're sending the translation to. Some organizations require translations to be signed by a solicitor, and in other cases you may need an Apostille. Always double check exactly what is required, as a signed or legalized translation tends to be considerably more expensive than a basic certified translation.