import decimal                               # Decimals
a=decimal.getcontext().copy()
print a

what is the useful of a.

I am answering assuming a novice.

"a" is a variable. Variable is a logical unit that keeps a value/string etc on computer's memory while executing a program. As an example if you are going to add 1 and 2 and get the answer, you should create a "variable" in memory say "a" and assign the value of 1 + 2 for "a"

In this code you have imported a library called decimal. It contains set of operations or methods. Then you perform some operations on it and assign the output to a variable called "a". Then you are trying to print it to console.

N.B : You should not use variable names like a,b. It is not a good practice. You should use a meaningful word.