An era is a period of time during which there is a specific set of active validators. Each era has 6 epochs (or sessions). During 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.

How long does an era last?

An era is 24 hours on Polkadot and 6 hours on Kusama (give or take a few seconds).

Where can I see what phase of the era?

The current phase of the era can be seen at the top right corner of the Polkadot-JS UI staking overview page or on the Overview page of the Staking Dashboard:

Polkadot-JS UI

Staking dashboard

(Hover the mouse over the graph to display the current era.)