Sizes descend in powers of ten: standard 100,000 units, mini 10,000, micro 1,000, nano 100. See standard-lot and micro-lot for the larger steps. On a dollar-quoted pair a nano lot is worth about one cent per pip.
Nano sizing exists so a very small account can still honour a sensible risk-per-trade instead of being forced into a position ten times too big. Some brokers achieve the same thing by quoting in cents rather than dollars, which is where the name cent account comes from.
Example: a $300 account risking 1% has $3 per trade. With a 50-pip stop the required pip value is $0.06, which is 6 nano lots. A single micro lot would risk $5, or 1.7% of the account.
Related: micro-lot, standard-lot, trade-size-units