These kinds of temperatures are generally produced in solar flares, powerful bursts of energy. (sciencedaily.com)
The other idea is that lightning is sparked when cosmic rays, powerful bursts of energy from space, deliver particles with enough energy to launch a strike. (sciencenewsforstudents.org)
When the meter is full, you can access the Synergy stage for your Pokemon you chose that will essentially make them a more powerful and faster version of them, allowing you to dish out major attacks, including a VERY powerful Burst Attack that will do some major damage to your opponent. (nintendoenthusiast.com)