Currently I'm reading about halting problem, after considering $H(\langle M,w\rangle)$ is machine which will solve halting problem, and then using machine $H$ they created a new machine $D$ and they run $H$ on input $\langle M, \langle M\rangle\rangle$. My question is what does it means or it's significance that giving a description of Turing machine as input what is significance of $\langle M,\langle M\rangle\rangle$?

what is logical reason behind giving description Turing machine as input ?