1. The document discusses principal component analysis (PCA) and explains how it can be used to determine the "true dimension" of vector data distributions. 2. PCA works by finding orthogonal bases (principal components) that best describe the variance in high-dimensional data, with the first principal component accounting for as much variance as possible. 3. The lengths of the principal component vectors indicate their importance, with longer vectors corresponding to more variance in the data. Analyzing the variances of the principal components can provide insight into the shape of the distribution and its true dimension.