The meaning is stated within the principle itself: the sponsors, developers, and users should be able to maintain a constant pace indefinitely.
The answer from nvoigt is on point and provides examples of sustainable and non sustainable approaches. There is just one thing I want to add to the existing answer, and that is to consider some context surrounding the Agile principles.
Agile showed up as an alternative to the documentation driven, heavyweight software development processes of the time (still there to this day), i.e. predictive approaches such as Waterfall, with fixed processes and steps, and most importantly, deadlines on when each phase was supposed to be finished.
Waterfall or predictive approaches have their place, but for most software development initiatives they don't work, simply because the methodology does not fit the nature of the product being developed with it.
So what tended to happen was that the early stages went at a reasonable pace, but the more you developed, the more you learned about what you were doing, and with new insight and understanding, it became obvious that what was estimated and planned at the beginning was not realistic. In other words, you would be missing the deadline.
So how was this solved? Acknowledge the error, stop, re-evaluate, re-estimate and decide on a new deadline extrapolated from the additional acquired knowledge?
Overtime. Crunch time. Death marches. Basically, putting more pressure on people to work more hours to still hit the fixed agreement committed at the beginning on the project when you knew the least about what you were building.
Choosing that approach causes people to burn out, make mistakes, take shortcuts, miss things, etc., which in turn decreases the quality of the product, which then has more bugs, which need to be fixed, which require more overtime, more stress, and more pressure, until the project either ends somehow, or people have enough and leave.
That's not sustainable! So, as a response to that, Agile methods promote sustainable development.