It’s hard to say anything about this without more details. What kind of plate is that, how is it loaded, which material model is used and where do you measure the stress ? Also please attach a stress contour plot (on the deformed model) if you can.
no details so just educated guess
could we be looking at pseudo-time? In other words it's not time but load (factor) and 1.0 being the full load (applied to the FEM). Some sort of non-linear analysis (no idea what kind of non linearity if any)