Let’s try to explain compression with a mathematical example:

You have the numbers 10, 35, 56 and 59. You form an average out of those values, which is 40. Now you give that average number to me. You tell me it’s made up of 4 numbers and ask me what those numbers were. I can form a guess, based on other examples you send me before and told me afterwards what numbers were in there. But I will at most randomly guess the right values by chance.

Compression is nothing different than this example except the mathematical numbers represent colors.