In Simple Words
Many beginners see the price of one whole bitcoin and assume they need that full amount to buy any.
They do not.
Bitcoin can be divided into much smaller units. The smallest unit is called a satoshi, or simply a sat.
This means you can buy a small fraction of a bitcoin without needing to buy a whole one.
The idea is similar to the way one euro or one dollar can be divided into smaller units — except bitcoin can be divided much further.