When you are challenged or under pressure, you are under stress. That is, you are subject to an external stressor. How well you cope with that stressor, whether you deal with it or give way, depends on your level of mental toughness.
Imagine a straight piece of wire (or a steel rod if you are feeling strong!). Apply an external force, a stressor, to the wire and it will bend. While it is subject to the force, the wire, internally, is under strain, but remove the force, and it returns to being straight. You can do this many times and the wire stays straight.
Now apply a greater force. When the force is big enough, the wire ‘gives way’. In engineering terms it goes ‘plastic’ and deforms. Now when you release the force, the wire remains bent to some degree.
But we know that wire varies in how much force it will bear before deforming. The springier the wire, the more force it will take and still return to straight.
There is a strong correlation between mental toughness and management seniority. The more senior you are, the higher your mental toughness score is likely to be. And to get to a senior role, you will typically need a good level of mental toughness.
Have a look at the link for a fuller explanation of what gives you ‘springiness’. (And try the above thought experiment for real with a paper clip or a wire coat hanger).
My best wishes, Paul