Heraldry is the profession, study, or art of the coat of arms. The coat of arms was originally created to differentiate between fighters in war, as much of the time they would have faces obscured by helms, and for obvious reasons it could be difficult to distinguish who was who. Since the original concept, it has broadened out into having meanings for almost any part of the arms.
Tinctures are the different hues on a coat of arms; in English heraldry, there are a number of rules such as metals not being atop metals or colours being atop colours. However, these don't have to be respected, and the special names are more often than not for show in Gielinor, any colours could be used.
"Metals" are not actual metal, simply the typically lighter tinctures.
Argent (white or silver)
Colours are the typically darker tinctures, containing the most tinctures.
Stains are the typically lesser-used tinctures in real life, alongside a few others.
Tenné (orange or brown)
Murrey (reddish purple)
Sanguine (blood red)
As well as the visual coat of arms itself, the blazon describes it in words; from the originally created designs, the arms of Lumbridge would be "azure, a chevron argent", the arms of Falador would be "azure, a mullet agent" (assuming there would not be a different term for the star of Saradomin within heraldry on Gielinor), the arms of Karamja would be "vert, a palm argent", the arms of Canfis would be "azure, a canis or", and so on.
The grammar of it is as follows; first noted is the field (background) and the colour or colours therein, which is always followed by a comma, before going on to the main charge, and then to any other charge there may be. If the charge is an animal, typically the attitude (position) is described.