Alpha Lock is one of the most well-known and effective ways of masking a layer’s content. Locking the transparency of a layer with Alpha Lock will let you draw or paint only within the boundaries of the already existing content on the locked layer.

To activate alpha lock on a layer, tap on the settings icon that sits on the bottom left corner of each layer thumbnail on your layer panel to invoke the layer settings menu. From Layer settings, toggle the Lock Alpha switch on. 

The alpha lock icon on the layer thumbnail of the Layers Panel, indicates that the Alpha Lock Mode is successfully activated on a layer. To disable Alpha Lock on a layer, open up the layer settings and toggle the Lock Alpha switch off.