What does this mean?

It means that it is raining hard/heavily.

Rain increases the power and (I think) accuracy of water-type moves. It also makes the electric move "Thunder" much more accurate.

kyogre's special abillity...

On serebii.net under the ability swift swim is says "When rainy, The Pokémon’s Speed doubles. However, Speed will not double on the turn weather becomes Heavy Rain."

If someone uses rain dance twice will the power of water moves greatly increase?

rain dance no rainy day.

Nonono.

Heavy rain is only on the route underneath the pokemansion.

If you us Rain Dance twice it will do nothing.

Rain Dance will not do anything if used more than once. The only way to get Heavy Rain is to go to the Route that supports it, or, i think, have a Kyogre out.

Heavy Rain, or rain in general, will make water type moves a little strongs, and water pokemon a little faster too. It'll also make electric moves stronger.

quote xiphos

Rain Dance will not do anything if used more than once. The only way to get Heavy Rain is to go to the Route that supports it, or, i think, have a Kyogre out.

Heavy Rain, or rain in general, will make water type moves a little strongs, and water pokemon a little faster too. It'll also make electric moves stronger.

False.

It only:
Makes Pokemon with the Ability Swift Swim faster.
Makes Pokemon with the ability Hydration get rid of status conditions every turn.
Makes Pokemon with the ability Rain dish regain 1/16th of their health every turn.
Makes Pokemon with the ability Dry Skin gain Hp every turn and will restore 1/4th of their hp if hit by a water attack.
Increases the base power of water moves by 50%
Decreases the base power of Fire moves by 50%
Solar Beam requires two turns to charge.
Thunder's accuracy increases to 100% and able to break through protect and Detect 25% of the time.
Castform transforms into a water type pokemon.
The move Weather ball becomes a water typed move and has a base power of 100.
Sythe sis, Morning Sun, and Moonlight only recover 25% of HP.

*boom* *boom* headshot