they say what doesn't kill you makes you stronger what exactly does that mean?

This should help you out although lot's of people interpret it in different ways.

What Doesn't Kill You Makes You Stronger - The best brain possible

It simply means that by overcoming difficulties in our lives -- whether physical, moral, or psychological -- we develop the necessary strength (and/or armour) to cope with situations we are yet to confront.

One of the problems with "helicopter parenting" is finding the balance between protecting our kids from harm and allowing them to take their knocks so they can build up their own strength.

From mistakes you gain wisdom . . . There is strength in wisdom.

Micro tears in muscle tissue grow back more larger and more dense . . . You are conditioning the muscle to adapt to what you are using it for. It is getting stronger.

Both boxers and martial artists  get micro fractures in their knuckles . . . Each time the bone heals those fractures, it get thicker and more dense. Career boxer's hands weigh more then they would normally if they hadn't conditioned them. Bones usually always grow back with significant calcium in the places they fractured or broke to make it stronger.

Certain things we go through can be shocking the first time we interact of see them . . . The mind has a way being de-sensitized regarding this, and it doesn't shock or paralyze us with fear after many exposures.

Anyone or all of these could fit as an answer.

Dear Thrice Gotcha,

I think that Rooster's link, the Nietzsche, is the actual explanation for this quote...plus Dozy's personal experience...however I have always leant toward the Hemingway version, from A FAREWELL TO ARMS:

"The world breaks everyone and afterward many are strong at the broken places. But those that will not break it kills. It kills the very good and the very gentle and the very brave impartially. If you are none of these you can be sure it will kill you too but there will be no special hurry."

If u ever go to prison as i had for two lousy years it will make u stonger. Their is no ac. No ice machines everything is for sale from ice to disenfectant

The more bad things you go through, the more you learn and the more you become stronger. (Mentally)