system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does placing a plastic bag over a credit card help a device read its magnetic stripe better?
gpt
The black rectangle on your card holds a number.  Actually it's a set of numbers separated by delimiters, but that's not important.  Just pretend it's one number.

If your card gets damaged, gaps appear in the electrical reading of the black strip, which means that inconsistencies appear in the number.  That's no good; that's cause for error detection to throw the whole thing out.

Putting a plastic bag around the card spreads out the electrical influence of each individual atom in the card.  This neutralizes (to some degree) degradation the card, because while the interaction with the reader is reduced as a whole, the relative reduction between the bad parts and the good parts are reduced.  Consequently, the machine can read it.

To simulate what's happening, open up Microsoft Paint (assuming you're on Windows).  Write, in huge size font.  a number.  Doesn't matter what.

Now scribble all over one or two of the characters.  I'll bet it it's hard to read the number!  Now zoom out until you can just barely make out the numbers.  I'll bet you can get the original number you wrote.

It's the same effect, but through electricity instead of your eyes.