What it means
Somebody sees your Instagram post, searches your name a week later, clicks a Google ad, then walks in on Saturday. Attribution is the set of rules that picks which of those gets the credit. Every platform uses its own rules, and every platform's rules favour that platform.
Why it matters to a small business
It explains the single most confusing thing in small-business marketing: your ad accounts claim more sales than your till recorded. They are not lying, they are double counting, because each one saw its own touch and claimed the sale. If you budget off the sum of what the platforms claim, you will overspend on whichever one is best at claiming.
In one month Meta reports 30 purchases, Google reports 22, and your own till recorded 38 orders in total.
30 plus 22 is 52 claimed against 38 real
At least 14 orders have been counted twice. The honest total is the one your own books show.
The bit people get wrong
The fix is not a better attribution model, it is a holdout. Comparing platform-reported numbers against each other only ever tells you which platform is more generous to itself. Turning something off for part of your audience tells you what it was actually adding.