The 'be-all-and-end-all' is now used to refer to a decisive life-changing event. It is usually used in the negative e.g. Your exams are important but are not the be-all-and-all.
The phrase comes from Macbeth (Act 1, Scene 7 4-5). King Duncan has arrived at the castle of his newly appointed Thane of Cawdor. The Macbeths are very hospitable - but do plan to kill the king in his bed to fulfil the prophecy of the witches (AKA the twisted sisters).
Now Macbeth is having second thoughts. He wonders whether this blow/ Might be the be-all and the end-all here. In other words, will murdering Duncan ensure a smooth transition to the long, peaceful and prosperous reign of King Macbeth?
By the end of his soliloquy, Macbeth seems to have come to his senses. He resolves that the plot 'shall go no further'. Then his wife has a quiet word...
See Ian McKellan & Judy Dench act out one of the key scenes in English literature here:
Getting Started with Macbeth - Teaching resources for Act One (£2.99)