What is pressing?
Pressing is the application of pressure and other techniques to improve the overall appearance, collectability, and saleability of a comic or magazine. Defects that can be reduced or eliminated entirely include: dents caused by a reader's finger and fingernails; bends and creases that do not break the cover's color, spine roll due to storage, reading, or simple aging; warping and waving caused by moisture (this can cause an appearance that looks like a ripple-cut potato chip).
Why do it to comics?
For all the obvious reasons: to improve the overall appearance by eliminating or reducing damage and defects and to increase its saleability.
What NOT to press
No comics or magazines with black mold anywhere on the covers or interior pages
No comics or magazines with brittle or loose pages