What does mean 'Expert Translators'?
Expert translators are translators specializing in a certain subject matter. Different than a 'generalist translator', an expert translator will usually work in a single 'vertical'.
Some examples of expert translators:
Most expert translators will have some kind of certification. A translator certification may come from a variety of organizations. One of the largest translation organizations in the world is the American Translators Association. Organizations outside the United States are also common and their certifications as valuable as those issued within the US.
Expert Translator Education
The education of the expert translator is a controversial subject. While some translation companies will require that the translator has formal education in their field (e.g.: a translator translating engineering documents should also be an engineer) most translation companies will require only substantial experience in their field.
Aside formal education in the area for which they specialize, formal education in language translation programs is also a big plus. Formal education in translation helps expert translators fine-tune their language skills while improving on quality control, terminology management, and other important aspects of the translation business.
Compensation of the Expert Translator
It may be counter-intuitive for some that expert translators will regularly decline translation assignments in "easy fields" outside of their specialties. By focusing on their specialties, expert translators are able to keep their knowledge up-to-date and command substantially higher 'per-word' rates for their work.
Therefore, by being picky about what the work they accept they are actually increasing both the job volume and per-word compensation for their work. That's why expert translators have an average income up to twice the earnings of those not specializing in a single vertical.