Masks let you hide or reveal parts of a layer — perfect for cutouts, reveals, or creative effects that need precision.
Steps to draw masks
Select the layer you want to mask.

Pick a shape tool from the top toolbar — Rectangle, Ellipse, or Pen Tool, depending on what shape you need.

Click and drag on the layer in your comp viewer and After Effects automatically creates a mask path for you.

One more thing
If you want to draw a mask and not accidentally create a new shape layer instead, make sure the layer is selected first before you use the shape or pen tool.
Related how-tos
How to animate a mask path in After Effects
How to feather a mask in After Effects
How to add motion blur in After Effects
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