Convert fonts to outlines
Text in a file normally refers to a font installed on your computer. If the print provider does not have that font, the text would be displayed incorrectly – with a substitute font or shifted lines.
To prevent this, you convert all text to outlines (also called “convert to curves”). Each letter becomes a fixed graphic shape, independent of the font. The text can no longer be edited afterwards, but it can also no longer “shift”.
What the function is called: in Adobe Illustrator Type → Create Outlines, in CorelDRAW Convert to Curves. In InDesign you export a PDF with embedded fonts. Tip: save an editable copy first.