Heralds make announcements, run ceremonies, and can help people choose suitable names and arms.
Voice heraldry - announcements at court and on the field
Voice heralds manage ceremonies at events, announce who is fighting on the tourney field and the result of each bout, and deliver news around event sites. The role is part master of ceremonies and part town crier.
Book heraldry - names and armory
Book heralds are available to help SCA members choose a name that could have been used before 1601, and design badges and devices.
Within the SCA it's typical to choose a name that could have belonged to someone from before 1601.
It is not compulsory, but many people choose a time and place to learn about, and develop a ‘persona’ of someone who might have lived in that time and place (but didn’t) who they wish to portray. This can help decide things like clothing to wear and stories to learn. This can also extend to creating a name for this ‘character’.
Heralds don't mind about your persona - if you have one - but if you'd like a name suitable for 13th-century Paris, or Seljuq Damascus, a herald might be able to help you find one. If you want to register that name as your unique name in all the SCA, a herald will help you with that process.
It is not uncommon in the SCA for a person to be referred to by their persona name rather than by their legal name.
Armory - devices and badges
Whether it's full medieval tourney gear with shields, surcoats, banners and pennons, or a subtle badge that identifies your cup as yours, or the livery or hausmarke that you share with your household, heraldry identifies people and is an opportunity for display.
Like names, heraldry can be registered uniquely to you within the world-wide SCA - although that doesn't grant any special rights in the modern world. If that's of interest to you, heralds can help you design armory that's likely to pass the registration requirements.