Well you probably don't get why I'm bawwwwwwww'ing with the ending, here's a brief explanation;
servants=heroes from the past/future/present, be it legends/people who really once existed.
Saber=irregular servant for the holy grail war, made a promise that she will obtain the holy grail, and because of that when she was about to die - time stops for her and she gets herself to be summoned as a servant (not her first time joining the war).
Saber=King Arthur (but in fate/stay night, Arthur is actually a girl pretending to be a guy during her life time because she is the King.)
Winning the holy grail war grants the winner 1 wish, but not really just that simple wish and then there goes the result, you are gonna sacrifice other people to be able to gain that wish, equivalent exchange. But shirou doesn't want that, and so does saber. So they decided to break the holy grail.
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Doing so means that saber will disappear as a servant and go back to her original timeline, and also time for her will flow again and she will die. But even knowing that fact, they still broke the grail...