sound effects, or more properly audio effects - are things like pitch shifts or reverb, you apply them onto a clip and they modify how said clip sounds. If you later decide you added too much of say reverb you can always lower the wetness of that effect, in the same way you'd see a layer of a picture halfway through you can now hear the sound effect halfway through!

dang, sorry i should've given more context to that, as a regular picture drawer i referred to image editing without realising others might not have the same point of reference for what opacity is

imagine you have a picture of pure white, you then add a layer of pure black, but you change the opacity of the black layer to 50%, which then shows your final picture as grey. Using more specific terms you changed the value of the alpha channel of the black layer to only be halfway visible.

This is what i was referring to as "opacity". Now with your example it'd work in the opposite way, at the risk of making it more confusing (i apologise in advance).

A CD song wouldn't be wet or dry.

You can take your CD song and apply an equaliser filter effect and make it sound like radio.

Now that filter has a wet to dry slider.

If you slide it all the way to dry you won't hear the changes the filter has made and you'll hear the original CD release quality. If you slide it all the way to wet you'll hear a modified version that sounds like radio, if you put it halfway through you'll hear, in a way, half of the effect you applied.

it's much easier to explain when you just have the thing in front of you and can hear the difference hah