A retrospective is a meeting where the team stop working and take a step back to review how well they are working and what they can do to improve.
It is generally done every sprint (or every two weeks if there is no sprinting going on).
The theory is easy – the team share their views on what went well in the last fortnight, what did not go well and what they should keep doing or change as a result.
In practice though, it often turns into a therapy session where people share opinions about the way the world is really bad (or even how the team is awesome) and then they wander off without taking on any action items.
So when your team have lost their mojo and the retros are getting stale maybe it is time to do a retro on the retro. Similarly, when you first start out as a team, it is often good to define what you might want to get from your retro.