Uncles are blockchain blocks found by a miner, when a different miner has already found another block for the corresponding place in the blockchain. They are called “stale blocks”. The parent of an Uncle is an ancestor of the inserting block, located at the tip of the blockchain. In contrast to the Bitcoin network, Ethereum rewards stale blocks as well in order to avoid to penalize miners with a bad connection to the network. This is less critical in the Bitcoin network, because the Block Time there is much higher (~10 minutes) than on the Ethereum network (aimed to ~15 seconds).