With opacity you can set the level of transparency for an effect or layer, as its name implies, thus making your setting or image more or less visible.
Opacity is itself a setting available for every Darktable tool and with Gimp’s layers.
Blend mode
The opacity setting is always used in combination with another setting named blend mode. There are dozens of blend modes, most of them are rarely used and by default opacity is just the transparency by itself, in that case the blend mode is simply called “normal”.
The most friendly way to understand blend modes is to try them, which can be easily done by pressing up and down arrows on your keyboard while the selection is in focus.
Here are some blend modes useful for photo editing, and a short description for them:
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Screen: tends to brighten the image
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Soft light: brighten the image, but much less than screen
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Vivid light: will strongly increase the contrast, thus darkening the image
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Hard light: will darken the image, though less than vivid light.
When to use:
Typically, when you are unsure about how much of an effect you want in the picture, slide opacity down to 0 to remove the effect, then raise it up again to 100% to see the difference, then settle to the amount you find more appealing.