An era is a period of time during which there is a specific set of active validators. Each era has 6 epochs (or sessions). Before the last epoch the active set of the next era is elected. And after the end of each era the rewards are calculated and are ready to be distributed to the validators and nominators. An era is 24 hours on Polkadot and 6 hours on Kusama (give or take a few seconds).

 

The current phase of the era can be seen at the top right corner of the Polkadot-JS UI staking overview page.