It's the duration of two consecutive blocks, under ethereum blockchain, time is divided into 12 second units, called slots. In each slot, a validator is going to be selected to propose a block. Case all the validator are functioning properly online a new block will be propose in each slot, Otherwise, there will be empty slots.

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Ethereum supports Proof of Stake and uses validators instead of miners, blocks are randomly selected by a validator to propagare this block to the network and consequently add it to the network through the assembly process. Unlike other blockchain, Blocks support of creation of smart contracts using a system of gas for the execution of them.

Ethereum Blocks

Ethereum Block Time

Blocks in Ethereum have a maximum size limit, which is set at 15 million gas, but can adjust to network demands up to a maximum of 30 million gas. All transactions included in a block must consume less gas than the block gas limit. This ensures that blocks cannot become too large, which would make it difficult for less powerful full nodes to keep up with the network. 

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