Question

What do the Natures mean?

Answer 1

Rather than just being a superficial personality, Natures actually
affect the growth of a Pokemon. Each nature increases one of its stats
by 10% and decreases one by 10% (by the time it reaches level 100).
Five natures increase and decrease the same stat and therefore have no
effect.

In most cases it is preferable to have a nature that decreases either
Attack or Special Attack for Pokemon whose strengths are the opposite
type of attack. Espeon, for example, favours Special moves, so it's
best to use a nature that decreases its Attack since it won't be used.

http://pokemondb.net/mechanics/natures

Answer 2

A nature increases a stat by 10% and decreases a stat by 10%.
For example:
- The nature timid boosts the speed stat by 10% but decreases the attack stat by 10%.
- The brave nature does the opposite by increasing attack by 10% while decreasing speed by 10%.

There are however certain natures that do nothing and they include: serious, docile, quirky, hardy, and bashful.
Source: http://bulbapedia.bulbagarden.net/wiki/Nature