A digital mockup shows where a print sits and roughly how it looks. A strike-off shows the truth: how the ink color reads on that fabric, how heavy the print feels, whether fine lines hold, and whether discharge or water-based ink behaves as expected on that dye lot. In fabric printing, the word also means a printed sample of the yardage sent for approval before bulk production.
Strike-offs earn their time on brand color matches, sim process or CMYK art, discharge prints, specialty inks, untried blanks and large orders where a mistake would be expensive to repeat. Screens have to be made and the press set up to produce one, so build it into your schedule.
Ask whether you will get a photo or the physical piece, what exactly you are approving (color, size, placement, hand), and how approval is recorded. A clear sign-off protects both sides. See the artwork guide and the sampling guide.