Currently I'm reading about the Halting problem. $H(\langle M,w\rangle)$ is a machine which will solve the Halting problem, and then using machine $H$ one creates a new machine $D$ and we run $H$ on input $\langle M, \langle M\rangle\rangle$.

My question is: What is the significance of giving a description of Turing machine as input as in $\langle M,\langle M\rangle\rangle$?

What is the logical reason behind giving the description of a Turing machine as input to $D$?