It’s a percentage that nominally increases as wealth goes up.
Poor people need to spend a higher percentage of their income meeting basic needs, so having them pay the same percentage as the wealthy puts a higher burden on the poor.
In top of that, the wealthy are able to put a higher percentage of their income in things like investments, which are taxed at a lower rate (to encourage investing in the economy over hoarding wealth), so a flat rate tax would be effectively a regressive tax.