Computational indistinguishability is like a game where you try to hide a toy under one of two cups and shuffle them around. Someone else has to guess which cup has the toy without seeing you put it under there.
In math, it's called the "security game" and the cups are like two sets of data. But instead of a toy, there are patterns or clues that show which set of data is "better." And instead of guessing, a computer tries to tell which set is the good one.
But it's hard to make the sets look different. It's like trying to make two cupcakes that look perfect and taste the same. If you can't tell the difference, they're computationally indistinguishable.
This is important because in computer security, you want to make it hard for people to figure out what your encrypted data is. If the encrypted data and random data look the same, it's harder to crack the code because you can't tell what's important and what's not.