It means different things to different people. For most people, retirement involves two big transitions- one psychological, and one financial. Sometimes people are ready (can’t wait, in fact!) for retirement, and sometimes they are not, and have the rug pulled out from under them by losing a job, or having a health issue pop up. Some people “try to retire” but go a little stir crazy and find they need or want to work some more, even if it is part-time, or on a volunteer basis. That is the psychological aspect of retirement- it can be complicated and stressful.
People who look forward to retiring and slide quite comfortably into their new routine often find that they are as busy or even busier than they were when they were working!
The other big transition is the financial one – you have stopped working, and that income is no longer coming.