Ethereum was using an algorithm called "Proof of Work" to secure the blockchain's state and it was using huge amount of energy as it literally was using hashing

Ethereum was using an algorithm called "Proof of Work" to secure the blockchain's state and it was using huge amount of energy as it literally was using hashing power of GPUs to verify/generate Ethereum "blocks" (immutable units containing transactions) to get Ether as rewards. The Merge changed this algorithm to "Proof of Stake", killing the GPU mining and the energy consumption completely (at least for Ethereum) and the new algorithm secures the network by giving incentives to Ether stakeholders (anyone can be, by the way) to secure the network instead of computing hashes on GPUs. This single event reduced world's energy usage by 0.2% overnight.