In Simple Words
Unlike traditional currencies, where new money can be created, Bitcoin was designed from the beginning with a fixed limit.
New bitcoin are created gradually through mining, but the amount created decreases over time.
This happens through the halving, where roughly every four years the reward received by miners is cut in half.
As a result, new bitcoin are created more and more slowly until the total supply eventually reaches 21 million.
This limited supply is one of Bitcoin’s fundamental characteristics.