Ethash is the name of the PoW algorithm used in Ethereum. Originally, this was proposed as the Dagger-Hashimoto algorithm, but much has changed since the first implementation, and the PoW algorithm has now evolved into what's known as Ethash now.
Similar to Bitcoin, the core idea behind mining is to find a nonce (a random arbitrary number) which once concatenated with the block header and hashed, results in a number that is lower than the current network difficulty level. Initially, the difficulty was low when Ethereum was new, and even CPU and single GPU mining was profitable to a certain extent, but that is no longer the case. Now either pooled mining is profitable, or large GPU mining farms are used for mining purposes.
Ethash is ...
