In Adobe After Effects the Generate effects presets allows you create an abstract shape form on a layer, for example with the Generate effects you can create audio waveform, grid, stroke, lightning, lens flare… All of these should help you create effects in one click instead of figuring out how to make the actual effects.

If this still seems vague to you it will make more sense when you see the preview gallery down below.

Where are the Generate effects presets located ?

Open the Effects & Presets window panel.
Click Generate to expand the folder
Click on any of the effects
Drag and Drop to your layer in your composition

Shortcut
To open the Effects & Presets panel use the shortcut below
PC: CTLR + 5
MAC: Command + 5

How To use the Generate Presets effects ?

In order to create a Generate effects you need to have layer, solid, image or video in your composition.

For specific generate effects such as Audio waveform an Audio Spectrum in addition to the layer you will need to have sound layer in your composition in order to have the effect to work.

Generate Effects preview gallery below

I used the default settings to show you an general overview of each generate effects but fill free to open the Effect Control panel to customize each effects.
Some of the example below are animated other are not it should allow you to get an idea.

4 color gradient Advanced Lighting After Effects
4 Color Gradient Advanced Lighting Audio Spectrum
beam effect CC Glue Gun
Audio Waveform Beam CC Glue Gun
CC light burst
CC Light Burst 2.5 CC Light Rays CC Light Sweep
CC Threads Generate cell pattern
CC Threads Cell Pattern Checkerboard
Circle Ellipse Eye Dropper Fill
Fill Fractal Gradient Ramp
Lens Flare
Grid Lens Flare Paint Bucket
Generate Radio wave Generate Stroke
Radio Waves Scribble Stroke
Write on
Vegas Write-on

Also here is a beautiful YouTube video that explain each of the Generate effects in After effects.

You are still looking for more information on the Generate Effects? please check the adobe website it goes over the specific settings of the each Generate Effects, or feel free to comment down below if you have a specific question.