When the ancient Greeks studied circles,they always got the same number whenever they did a couple of things,and that number they called pi.
What they did : measured the area of a circle and its radius. Then divided the Area by the square of the radius. Or,if you like ; A/(r^2). They always got the same number,pi. You will know this as the formula for the area of a circle A = pi X (r^2)
They also got the same number if they measured the circumference and divided it by double the radius,this gives the formula for the circumference of a circle as circumference = 2pi X radius.
It did not matter what size the circles they measured were,they always got the same answer,and so they realised that the relationships were constant -,and pi is one of the most well-known constants in maths and physics. NASA has computed its value to hundreds of decimal places,as pi is a vital constant in working out problems for space travel.