A blazon is a specialised description of a shield. To illustrate how to decode them consider the blazon Argent on a fess sable three bezants between three fleur-de-lys gules (Thwaites).
The first step is always to tell us about the background.
Argent... This tells us the background or field is silver or white.
The colours used commonly are: argent=silver/white; gules=red; azure=blue; or=gold/yellow; sable=black; vert=green. For a full list see Colours.
Next we place any major features, such as a fess (the horizontal band).
Argent on a fess sable... On the white background place a fess sable (black). Since the blazon contains the words on a we know there is more to come.
Argent on a fess sable three bezants...Now we find out what is on the fess: three bezants, which are gold circles or roundles representing gold coins (the name suggests they were Byzantine coins).
Then we place any other charges, in this case three fleur-de-lys (stylised flowers) gules (red).
Argent on a fess sable three bezants between three fleur-de-lys gules. And finally the blazon tells us what it is that the fess-plus-bezants is between: three fleur-de-lys gules.
You will notice that the object (background, fess or charge such as the fleur-de-lys) is named, then its colour is stated. Also, when there are three charges they are understood to be placed 2 above and 1 below.
Or a cross patonce sable (Vescy). This is a very simple blazon, telling us the background is or, that is gold/yellow, and the only charge is a cross sable or black. The patonce defines the shape of the cross, in this case one with the arms widening out and splaying into three.
Argent a chevron between three lions heads erased gules (Rocliff). The background field is argent, i.e. silver or white, on which the first or major charge is a chevron, and the secondary charges are three lions heads. As both the chevron and the heads are gules (=red) the colour is only stated once. The term erased describes the ragged edges of the heads; had they been cut off straight they would be couped.
Quarterly azure and argent, in the first quarter a fleur-de-lys or (Metham).