Channels
Channels are the separate color components your image is built from, usually red, green and blue. Each channel is a grayscale version showing how much of that color sits where. You use them for advanced selections and color editing.
Open the channels panel to view and use the individual RGB channels. Often one channel has the most contrast exactly where you want to select, like hair against a sky, which makes a handy basis for a selection. Frequency separation and color corrections also lean on channel thinking. Tip: duplicate a high-contrast channel and use it as a starting point for a mask.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.