Not one thing called 'stablecoin'
The word stablecoin describes an outcome, a token that holds a roughly constant value against some reference (almost always the US dollar), not a single mechanism. In practice there are three structurally different ways engineers have tried to produce that outcome. Fiat-collateralized stablecoins are backed by real dollars (or dollar-equivalent assets) sitting in a bank account or similar custodial arrangement off-chain, with the token representing a claim on that reserve. Crypto-collateralized stablecoins are backed by volatile crypto assets locked in an on-chain smart contract, over-collateralized so that normal price swings don't threaten the peg. Algorithmic stablecoins try to hold a peg through supply adjustments and market incentives, with little or no collateral backing the token directly. These aren't minor implementation variations, they represent different answers to the question of where trust actually lives.
Understanding which category a stablecoin belongs to is the single most useful piece of information about it, because it predicts both how it maintains its peg day to day and how it's likely to fail under stress. A custodial dollar-backed token, an over-collateralized crypto vault, and an algorithmic supply-rebalancing token can all trade at approximately one dollar on any given day while resting on completely different foundations. This lesson starts with the first and conceptually simplest category, and the next three lessons build out the other two plus a systematic comparison of how each one breaks.