Drift is the difference between where an image is printed, and how accurate the cut is to the location of the print. Both printing and cutting need to be perfectly accurate in order to have your cards, boxes, tiles, etc come out looking perfectly. That said, there is no such thing as perfect. For this reason, your images need to include bleed. However, your design can also make drift look more or less apparent.
Imagine you have designed a card with a 1.5mm (1/16") border, a 3mm (1/8") border, or no border at all.
Now imagine that there's even 0.5mm (0.02") of drift. This is what your cards would end up looking like:
Even that tiny amount of drift is hugely noticeable on the cards with the 1.5mm border. It's less noticeable on the 3mm border, and not noticeable at all when you have no border. And remember that the industry standard is to have a 3mm bleed, so that means that there could be up to 3mm of drift within your printing, not just 0.5mm. Every printer strives to do better than 3mm, but no process is perfect all the time. Therefore your design needs to take that imperfection into account. For example, here's what a drift of 1 mm changes things.
All printing processes have drift. So design your components to take drift into account for the best possible result.
Watch the video below for more details.