What is Alpha lock in procreate?

Alpha Lock gives you the ability to draw within that layer’s shape; this command is ideal for drawing within the shape’s boundaries. While many rely on the Alpha Lock command to apply textures, shadows, and highlights, Alpha Lock is extremely useful to quickly change the layer’s fill colors.

Which option is used to lock a layer?

Locking your layers prevents them from being changed. To lock a layer, select it in the Layers panel and select one or more of the lock options at the top of the Layers panel. You can also choose Layer→Lock Layers or select Lock Layers from the Layers panel menu.