What exactly does Focus Energy do?

Question

In Pokemon, there is a move called Focus Energy. The move describes:

The user takes a deep breath and focuses so that critical hits land more easily.

But what exactly does this mean? How much easier is it for critical hits to land?

Answer 1

According to the wiki from Gen 4+,

Focus Energy now increases the user's critical hit ratio by two stages instead of one.

where critical hit stages have the following crit chances. -source

• Stage 0 -- 6.25% (base rate)
• Stage 1 -- 12.5%
• Stage 2 -- 50%
• Stage 3+ -- 100%

Answer 2

I know you tagged sixth generation, but it is worth pointing out that Focus Energy was glitched in the first generation and actually quartered your critical hit ratio instead of quadrupling it. It works in stadium correctly though.

In generation 2 & 3, Focus Energy raised your critical hit ratio by 1 stage. Starting in generation 4, it raises your critical hit ratio by 2 stages.

See Focus Energy on Bulbapedia.

I figure it's relevant since RBY have been released on eShop and you'll be able to move those Pokemon to Sun and Moon.