Power spectrum is a powerful tool for analyzing structures in the universe from the very smallest to the very largest. It is nothing but the square of the amplitude of the Fourier transform of a dataset, mathematically

$$ P(k) = |F(k)|^2 $$

where $F(k)$ is the Fourier transform of a function $f(\theta)$ and $k\sim \theta^{-1}$ is the Fourier scale or frequency corresponding to the variable $\theta$. A power spectrum gives the power $P(k)$ at different scales $k$.