It's a weather condition. When it's sunny, Solar Beam skips the charge phase and can be used in rapid succession. Weather effects also effect certain moves like so:
In Hail:
All non-Ice type Pokémon are damaged each turn
Bizzard's accuracy is increased to 100%, being able to break through Protect & Detect 25% of the time
Synthesis. Morning Sun & Moonlight recover 25% of user's HP
Pokémon with the Ice Body Ability have 1/16th of their max HP recovered each turn
Pokémon with the Snow Cloak Ability have their evasion boosted by 20%. Outside battle in 216 & 217, if head of the party, the wild encounter rate is decreased by 50%
Castform transforms into a Ice type Pokémon
The attack Weather Ball's Power doubles and becomes Ice Type
In Rain:
Water Type move's Base Power is increased by 50%
Fire Type move's Base Power is decreased by 50%
Solar Beam requires two turns of charge
Thunder's accuracy is increased to 100%, being able to break through Protect & Detect 25% of the time
Gale's accuracy is increased to 100%, being able to break through Protect & Detect 25% of the time
Synthesis. Morning Sun & Moonlight recover 25% of user's HP
Pokémon with the Dry Skin Ability recovers HP every turn and 1/4 its HP when hit by a Water Attack
Pokémon with the Hydration Ability are cured from status afflictions whenever it rains
Pokémon with the Rain Dish Ability have 1/16th of their max HP recovered each turn
Pokémon with the Swift Swim Ability have their Speed raised
Castform transforms into a Water type Pokémon
The attack Weather Ball's Power doubles and becomes Water Type
In Sunny:
Fire Type move's Base Power is increased by 50%
Water Type move's Base Power is decreased by 50%
Solar Beam does not require charging
Thunder's accuracy is reduced to 50%
Gale's accuracy is reduced to 50%
Synthesis. Morning Sun & Moonlight recover 100% of user's HP
Pokémon with the Chlorophyll Ability have their Speed raised
Pokémon with the Dry Skin Ability received double damage from Fire attacks and loses HP each turn
Cherrim's Flower Gift Ability increases it's Attack & Special Attack and changes it's form
Pokémon with the Leaf Guard Ability are protected from status afflictions
Pokémon with the Solar Power Ability have their Special Attack raised but HP lowered
Castform transforms into a Fire type Pokémon
The attack Weather Ball's Power doubles and becomes Fire Type
In Sand Storm:
All non-Rock, Ground or Steel type Pokémon are damaged each turn
Rock type Pokémon's Special Defense is increased by 50%
Solar Beam power is reduced
Thunder's accuracy is reduced to 50%
Synthesis. Morning Sun & Moonlight recover 25% of user's HP
Pokémon with the Sand Veil Ability have their evasion boosted by 20%. Outside battle in 111 & 228, if head of the party, the wild encounter rate is decreased by 50%
Pokémon with the Sand Strength Ability have their Ground, Steel & Rock move strength increased by one stage
The Weather Ball attack's Power doubles and becomes Rock Type
EDIT: I'm so bad. Look at me. I should be doing homework and instead I'm here talking about Pokemon. Bad me. *wristslap*