A stablecoin is a digital coin designed to always be worth around $1. The two big ones are USDC and USDT (Tether). People use them to move dollars across the world fast and cheap, or to park money inside crypto without exposure to price swings.
How does a coin stay at $1? The serious ones (USDC, USDT) are backed by real dollars and short-term US Treasuries held by the issuing company. For every USDC in circulation, in theory, there's a real dollar in a real bank somewhere. That's audited regularly for USDC. USDT's transparency is more debated.
There used to be "algorithmic stablecoins" that tried to stay at $1 with code instead of real reserves. They worked until they didn't. UST collapsed to zero in 2022 and wiped out billions. Stick with USDC or USDT for now.