In most PoS networks, new blocks create new tokens, which are paid out to validators and delegators and are referred to as block rewards. These block rewards increase the token supply of the network, hence, they have an inflationary character. A network's inflation rate describes the annualized growth rate of the total current supply at any point in time.
Token holders who do not participate in staking see their funds getting diluted over time by the inflation rate of the respective network. Delegators can secure themselves from getting diluted by staking their tokens.