The number Pi, denoted by the Greek letter π - pronounced 'pie', is one of the most common constants in all of mathematics. It is the circumference of any circle, divided by its diameter. Nobody knows its exact value, because no matter how many digits you calculate it to, the number never ends. For most practical uses, you can assume it is 3.142.
Some people have written computer programs and calculated it to an astonishing accuracy. For example some have calculated pi to 200 million digits.
Most calculators have a button to enter the value of Pi directly, so you don't usually have to remember all the digits. But it's handy to remember pi = 3.142 anyway.
Using the calculator Pi button is better, because it inputs Pi to the most number of decimal places that the calculator is capable of. This will result in your calculations having the best possible accuracy.
If you just need a rough value for Pi you can assume it is 22/7, but this is only accurate to two decimal places. However it may be accurate enough for some uses when you don't have a calculator handy.
An even more accurate approximation is 355/113. This is corect to 6 decimal places.