What is the .modulus() function doing?

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I came across this tutorial from Sage that I couldn't get.

Below is the code.

k.<a> = GF(2^1024)
k.modulus()
Output: x^1024 + x^19 + x^6 + x + 1

What is going on here?

I understand that the polynomials' coefficients are either 0 or 1, and the degree of the polynomial is not more than 1024, but how does Sage assign them? And also, what is the modulus of the polynomial? I have never came before such a term and I couldn't find any such concepts on the internet.

Thanks for the help in advance.