This is part of the study of geometry.
Yes, a circle is technically a polygon "with a lot of sides", but that "a lot" would actually lean more towards the concept of infinity; a circle is a polygon with an infinitely large amount of sides.
By reducing the size of the measurement, or increasing the accuracy (in this case by increasing sides), one gets a result closer to the actual one.
This concept has been reused for the creation of other fields of mathematics, such as calculus.