What is stress? What is strain?

The answer to the second question is part of the answer to the first question.
Most experts will advise to run your web at 10% of the tension that would damage the web, a safety factor of 10:1.

Definition: Stress is a force over an area (units are lbs/in2 or psi, N/m2 or Pa)

Stress and pressure have the same units. Tensile stress tends to elongate something. Compressive stress (a.k.a. pressure) tends to shorten something.

Definition: Strain is a shape change, usually described as a fraction or percent change calculated from dividing the change in a dimension by its original, unstressed dimension.

When you tension a web, it gets longer in the machine direction. If you stretch a 250mm (10-inch) sample to 252.5mm (10.1 inches), you have strained it 1 percent.

straining elongation