Basically, the ice crystals in a thunderstorm rub up against each other and generate a massive static charge. Once this static charge becomes large enough, it discharges to the earth. Thunder happens because the air gets superheated and essentially explodes when the lightning discharges, generating a shockwave which you hear as the thunder.
Tristinmathemusician t1_jecqkqh wrote
Reply to Eli5: how does thunder and lightning happen? by Glubygluby
Basically, the ice crystals in a thunderstorm rub up against each other and generate a massive static charge. Once this static charge becomes large enough, it discharges to the earth. Thunder happens because the air gets superheated and essentially explodes when the lightning discharges, generating a shockwave which you hear as the thunder.