It's instructions for a stitch pattern. So the pattern has 8 stitches to make the design, the + 3 balances it out to look right for flat knitting so it might be 64 stitches +3. Eight repeats of an 8 stitch pattern 8X8 = 64 + 3. Or it could be 40 + 3; 5 x 8 =40 + 3.
Usually it will tell you to cast on a multiple of 8 plus 3.
so if it 8x8=64+3=67. the +3 is it add to every 64 stitches?
No, only if 64 stitches is close to the width you want. Start with 3 stitches then keep adding groups of 8 until you have the width (or length) of fabric you want.
If you need 100 stitches for the size you want then 12*8 =96 and then +3 makes it 99. If that isn't long enough you could go to 107 which is 99 plus 8 more stitches. Do you see the pattern now? You only add the 3 once, not to every group of 64 stitches.
The point is that there are 8 stitches in the repeating stitch pattern and 3 stitches added for the selvage edges. If you want it wider you can increase by 8, 16, 24, 32, 40, etc. Or if you want it narrower then, decrease by similar numbers based on the gauge measured on your swatch.
Scarves, dishcloths, blankets are the type of items that you don't need to exactly match the gauge of the designer. Or you make choose to may yours wider or more narrow than the designer.
Master of Crochet, apprentice of Knit