Light nodes are usually pieces of software connected to the Bitcoin Network (e.g wallets). These nodes perform similar functions to full nodes, in the sense that they need to process transactions to operate, but cannot enforce any rule changes on the network. As such, they cannot participate in the consensus mechanism of the blockchain.
Instead, they use the data of full nodes to check the authenticity of transactions in order to interact with the blockchain. Compared to their full node counterparts, these nodes only need to hold part of the relevant transaction history (often referred to as block headers). Although this makes them more cost-effective, they are still dependent on full nodes that ultimately need to establish a final consensus.