The \substack{} makes it so that the two lines separated by \\ are stacked above each other, as explained in

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Intro to Stacking & Layering. The first line/layer is made light blue with the \color{lightblue} tag, and made very large font size with \Huge. It's also made bold with \textbf{}, which automatically triggers text mode. The second layer is \Huge and default color, and we trigger \text{} mode, as explained in

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Intro to LaTeX fonts . The second line, the black Hello, is shifted up 1em units to overlap with the blue slightly using a [-1em] right after the line break \\.