A fiat-pegged cryptocurrency is a crypto token designed to keep a fixed value against a fiat currency, most often one US dollar per token. Today these are almost always called stablecoins, and the full explanation is on that page. Most are backed one-for-one by reserves of cash and short-term government bonds held by the issuer, such as Tether (USDT) and USDC.
A different design, the algorithmic stablecoin, tries to hold the peg through code rather than reserves.