Inflation is the increase of the money supply within an economy, benefitting those who get to spend the new money first the most and then creating a "ripple effect" of decreasing utility around those entry points as said new money is divided among more people, increasing prices along the way by decreasing the value of each individual unit of currency due to there being more of it.

When it reaches people who don't benefit from increased money supply at all, mostly because their income isn't directly based on the price of goods they sell but on things like fixed salaries or pensions, they are forced to simply pay more for the articles of daily life using the same absolute amount money they had before which has now decreased in value, allowing them to purchase less, i.e. lowering their "purchasing power", making them poorer in real terms.