In my class we are studying pi. I don't understand it. Could someone please explain it to me?

Pi (from the greek word that means 'periphery') is simply a ratio, or a fraction of the circumference (distance around a circle) to its diameter (distance across a circle)  or

1)  circumference / diameter, or c/d

One of the properties of all circles, no matter how large or small, is that the fraction of circumference and diameter will always reduce to 22/7, so if the distance around a circle is measured as 22, the diameter of that same circle is always 7. If the diameter is 14, the circumference is always 44, and so on.  However, since 22/7 is simply a fraction, although a very special one,  it can be represented in decimal form

2)  22/7 = 3.1416....

Since 22 divided by 7 doesn't come out evenly, the number goes on forever, but rounding it to 4 digits is usually good enough for calculations. So this 22/7 fraction, or this 3.1416 decimal number, is given the special name of Pi.