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"Settings" is the grey gear icon--it should be on your iPod's home screen. This is located right underneath the "General" tab. Auto-brightness brightens or dims your display based on how much ambient light your iPod detects, but it's a huge battery-drainer. This will dim your display. You can adjust your display brightness at any time from the quick-access menu, which is accessible by swiping up from the bottom of the screen.
Open the Settings app. Tap the "Display & Brightness" tab. Slide the switch next to "Auto-Brightness" to the "Off" position. Slide your brightness adjustment all the way to the left. Close your Settings app.