What does 声優 Seiyuu mean?

Seiyuu means voice actor. The translation is precise, which makes this a different kind of entry: the word is not misunderstood, the job is.

The characters

声 (sei) is voice. 優 (yuu) means actor, and also carries senses of gentleness and superiority; it is the same character in the general word for a stage or screen actor. So the compound sits inside the acting profession rather than beside it, which is the first small signal that the role is not thought of as a technical speciality.

What the job includes

Recording, obviously. Also, routinely: singing in character, releasing music, appearing at live events, hosting radio programmes, fronting merchandise, and being photographed. Performers accumulate followings that travel with them between productions, and casting is discussed publicly in a way that would be unusual elsewhere.

None of that is implied by the English phrase. Voice actor in English describes work done in a booth by someone the audience is unlikely to recognise, and the prestige attaches to the character rather than the performer.

Why the difference exists

Volume and continuity. A production system making a large quantity of animation continuously supports a professional class specialised in it, with training routes, agencies and a trade press. Where animation is a smaller share of the industry, voice work tends to be something actors do between other jobs, or something celebrities are hired into for a film.

The vocabulary follows the structure. Japanese has a settled occupational term because there is a settled occupation.

The translation trap

This is the inverse of most entries here. Elsewhere the English word is too narrow or the wrong size; here it is exactly right and still communicates the wrong thing, because the two languages attach different social facts to the same description.

A subtitle rendering it as voice actor is correct and cannot convey that a character's performer might be selling out a venue that weekend. That is not a translation failure. It is a limit on what translation can do.