ToME: the Tales of Maj'Eyal

So I got a Copper Amulet of mastery (0.10 Technique / Combat-training)

What does this do?

I believe - it increases the worth of putting points into skills of that category.

Above each category is a number, usually 0.90 or 1.00 for skills that are central to your character, 1.30 for skills that the class focuses in. These seem to dictate how effective those skills are. For instance I notice trap detection for rogues gives more detection power than other classes at the same level.

I could be wrong - this might be the effect of the attributes.

You're spot on

Talent type mastery increase the effectiveness of talents.
A fireflash with a mastery of 1.3 will do more damage than with a mastery of 1