Dear Colleagues

I am struggling to understand stratified cox regression ?

In Google it says: The “stratified Cox model” is a modification of the Cox proportional hazards (PH) model that allows for control by “stratification” of a predictor that does not satisfy the PH assumption

However, I dont understand what stratification do?

My understanding is that the variable we stratify on is not included in the cox Re model? Is this correct?

So what happens then? Is a cox regression runs when this variable we stratify on is negative and run once again when it is positive ? And then the hazard ratios are averages of all the models?

If this is wrong, so what happens?

What if the variable we stratify on is really important factor for predicting survival?

How to interpret it in this case?