A set of transactions confirmed on the blockchain. Each block builds on the blocks before it.
When a miner confirms a set of transactions it is known as a block.
As soon as one block is found, it is broadcasted to the network and propagates to the other miners. At this time the miners begin searching for the next block. After a transaction is included in a block it's confirmation count increase by 1, with each block found after adding another confirmation to it.
The more confirmations a transaction has, the more secure it is. On most blockchains payments can be considered sufficiently secure after 0 to 10 confirmations - depending on the value.