To expand on what @johnpowell said, a sales tax is a “regressive tax” because the value cost of the imposed tax is greater to poor people than it is to rich people. The tax on a $5 sandwich is a greater percentage of the poor person’s budget than it is a rich person’s budget, because the rich person’s budget is much larger. Because he’s rich.
Think of it like a speeding ticket. $200 is an effective deterrent to keep me from speeding in my $500 rustbucket, because I’m broke and I can barely afford to keep the damn thing on the road. But if I just bought a $250,000 sports car (and believe me, I didn’t), that $200 isn’t really an effective deterrent any more. It’s such a small percentage of my income that I speed with impunity.
Sales tax works in an analogous way: you’re imposing the same costs across the board, but the value of those costs is different to different people.