A block reward in blockchain technology refers to the amount of cryptocurrency or tokens that are given to the miner or validator who successfully adds a new block to the blockchain. This reward serves as an incentive for nodes in the network to compete to validate transactions and add them to the blockchain in a secure and decentralized manner.
The amount of the block reward varies depending on the blockchain's consensus mechanism and its specific design, but it typically includes a combination of newly created coins and transaction fees. The new coins serve as an inflationary mechanism to incentivize early adopters, while transaction fees provide a source of revenue for block validators as the number of coins in circulation approaches the maximum supply.
The block reward is an important part of the security mechanism of a blockchain. By offering a reward for validating transactions, the blockchain is able to attract a sufficient number of nodes to secure the network, prevent double-spending, and ensure that all transactions are properly processed.
Over time, the block reward decreases as the total supply of coins approaches the maximum supply, and the blockchain must rely on transaction fees as the primary source of revenue for block validators. The reduction in the block reward also helps to control the inflation rate of the cryptocurrency and maintain its value over time.

Gas fees in Ethereum: In Ethereum, gas fees are paid by users to execute transactions on the network. Miners who validate these transactions and add them to the blockchain are rewarded with a portion of these gas fees, similar to a block reward.
Forging rewards in NXT: In the NXT blockchain, forgers (similar to block producers) were rewarded with a fixed amount of NXT coins for each new block they produced. These rewards were meant to incentivize forgers to secure the network and validate transactions.
Masternode rewards in Dash: In the Dash blockchain, masternodes are responsible for verifying transactions and maintaining the network's security. They receive rewards in the form of Dash coins for their work, which is similar to a block reward in other blockchain systems. The reward is distributed to masternodes based on their contribution to the network, which includes factors such as the number of Dash coins they hold and the length of time they've been active on the network.

A block reward in a blockchain can be thought of as a prize given to miners for solving a complex puzzle. Just like how a person might get a prize for solving a difficult crossword puzzle, miners receive a reward for adding a block of transactions to the blockchain. This reward incentivizes miners to use their computing power to validate and secure the network, keeping it running smoothly and reliably. The reward can be in the form of cryptocurrency, such as Bitcoin, and its value is determined by the rules of the particular blockchain.