“Rounds” are essentially Olympia blocks. If each round were full of transactions (which, of course, is unlikely to start), then the network’s “rounds per second” rate, multiplied by the number of transactions in each round, would give you a TPS number for the Radix Public Network.
The rounds/second number can be thought of as the pulse of the network that sets the length of an epoch since an epoch is a fixed number of rounds in length. 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.