In this introductory talk, we will introduce the important tool which called theta correspondence in the representation theory and the Plancherel decomposition in the Harmonic analysis. Sakellaridis and Venkatesh have proposed a conjecture related to Plancherel decomposition of $L^2(X)$ in terms of the dual group $G_X^{\vee}$ and the distinguished morphism $\iota$. In this talk, we are concerned about the case when $\iota=\theta$ is represented by theta correspondece.