It's really smooth and my friend used it in a video but I can't seem to find it, anyone know what it's called ?
If you're talking about After Effects then it wouldn't be an effect you apply, it would likely be a preset that allows you to fade in and out. If you open the Effects & Presets Panel and twirl down the Presets section and then Behaviors, you'll see "Fade In+Out - frames" or "Fade In+Out - msec" (or you can just search for "Fade" in the panel). Applying this preset to a clip will act as an effect and you can go into the Effects panel and dial in the amount of frames you want your layer to fade in and out.
In TV-Land where I work, we call 'em dissolves, wherein an image's opacity starts at zero -- i.e. invisible -- and goes to fully opaque over a given duration. Or from fully opaque to invisible.
In AE-Land it's called "animating opacity". You do it with keyframes on the opacity property, and it's dead simple.