What are rounds in the context of ciphers?
Many block ciphers are defined by specifying a round and then running that specification multiple times. For example, in AES, a round consists of the operations SubBytes, ShiftRows, MixColumns, AddRoundKey. That is one round and, to get AES, you run that multiple times (plus some setup and some post-processing).

Thus a round is defined by each cipher and typically consists of a number of building blocks that are composed together to create a function that is run multiple times.