Given a matrix $\mathbf X \to X_{m}^{\phantom{m}n}$, we can decompose it into three matrices

where $D_{k}^{\phantom{k}l}$ is diagonal.

Here we actually have $\mathbf U$ being constructed by the eigenvectors of $\mathbf X \mathbf X^{\mathrm T}$, while $\mathbf V$ is being cunstructed by the eigenvectors of $\mathbf X^{\mathrm T} \mathbf X$ (which is also the reason we keep the transpose).

I find this slide from Christoph Freudenthaler very useful. The original slide has been added as a reference to this article.