“Rounds” are the equivalent to blocks on Radix’s Babylon Mainnet. If each round were full of transactions, then the network’s “rounds per second” rate, multiplied by the number of transactions in each round, would give you the number of transactions per second (TPS) on the Radix Public Network.
The rounds/second number can be thought of as the pulse of the network. It’s determined by how fast the network can reach consensus – subject to latency, timeouts, etc. One of the cool things about the 3-phase consensus we’re using for Olympia is that it can go as fast as latency, timeouts, etc., allow, rather than having a fixed duration like, say, 2-phase Tendermint.