The crisis is the choice the MC has to make which is propelled by the biggest/current progressive complication. It happens within a progressive complication, and is one of two choices in essence:

THE BEST BAD CHOICE
or
GOOD FOR ME = BAD FOR SOMEONE ELSE

The Best Bad Choice is something of a choice of lesser evils.

Ex. If I jump I might die, but I might live, and if I stay I will probably definitely die.
Or
If I choose this it will be bad for me, but if I don't it will be even worse for the world.

Literary Example
Harry Potter has to choose death by Voldemort in order to save the world. (This is the MC crisis for this book, as well as for the series, and the point where Harry truly understands what it means to be a leader as well as his destiny)

The Good for me = Bad for them choice is usually one which brings more morality issues into play (though not necessarily). The reason is the "them" in this instance is usually someone the MC cares about and doesn't want to hurt.

Ex. My best friend and I are both in love with the same guy, if I go after him it might mean true love for me but it will devastate my friend and possibly ruin our relationship. If I don't go after him, I may be passing up my only chance at true love.

Literary Example
Elizabeth Bennett must choose whether or not to out Mr. Wickham's character to her family. To do so would shame him, but (probably) would have prevented her sister from eloping with him.  She chose not to and her chances at happiness were nearly ruined.

Now, each scene, act, and the global story will all have their crisis points. Each character arc (if there are multiple MCs will also have their own crisis.

Each MC and scene/act/global story arc should have each of these elements as I showed in earlier chapters.  Which means, there will be a lot of crises.

The Crisis is the major complication that turns that scene/act/global story arc. At the global crisis, it may be (probably) the crisis of the second act as well, and the MC crisis for sure.  It is at this point where the character should be able to realize their main flaw and (if writing a hero's journey) choose to overcome it.

The action proceeding directly from this choice is the Climax. I'll go into this more in the next chapter.

Each crisis should push the character deeper into the story toward the inevitable ending with each successive choice they make. It can be as simple as choosing to read a letter, or it could be refusing a marriage proposal. But whatever it is, it has to directly impact your character's arc.

Literary Example
The Crisis of Act II (and the global crisis) in Pride & Prejudice is the moment Mr. Darcy gives Elizabeth the letter and she chooses to read it. Some might say it's the moment where she refuses Mr. Darcy, but I think it's this point personally. The point of refusal is his crisis, which leads him to begin to rectify the situation, starting with the letter. Elizabeth is the MC and she is still wallowing in her own pride and prejudice until the point where she chooses to read the letter and sees how very unjust she has been. 
From that point she takes steps to change her ways.

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If this makes sense so far please consider voting on this chapter!

If you have questions, please don't hesitate to ask. I'm going to do some Q/A chapters later on so if you've got a particular area you want help on or more information please let me know and I'll make sure to address it in a later chapter.

Your engagement is what makes this book better so I appreciate all comments and suggestions.