The target price a stablecoin is designed to hold — usually 1 unit of the reference asset per token.
A peg is the target price a stablecoin is designed to maintain. Most stablecoins peg 1:1 to a fiat currency: 1 USDT is meant to always equal 1 US dollar. The peg is maintained through backing (reserves you can redeem against), arbitrage incentives, algorithmic supply adjustments, or a combination. A well-designed peg holds within a few basis points of target; a poorly designed peg breaks.