Its just an somewhat overused reference to the ability to be able to accept what is, rather than burn vital energy fighting non-ideal situations that you cant actually do anything about.
Example. Your in line at the bank and get impatient, most people will stand there think "fucking line, Im bored, fuck my life, how long is that person going to take!? grrrrr I hate waiting, Id rather be somewhere else right now!!" etc etc.
All of that is a complete waste of time and is just succeding in stressing you out. So whats the point? It is what it is

The line is there. So accept it..... and think of something else while you wait, or better still, observe and think of nothing at all
Stress kills you know, eliminating it is optimal. Allowing trivial stress is madness