When you create a new document to work on, the default color may be white and the name is background. You may want to edit this layer and add color and other effects. You can unlock the layer by double-clicking on it. A layer dialogue box will appear and the layer will be name Layer 0 unless you change the name. You can then edit the color and add other effects to the layer. In this case, you can edit the layer to get it to fit in with your project.
Layers can be unlocked partially to allow certain edits to be done. This partial unlocking will protect the image from being accidentally edited in areas that you don’t need to be done. To allow partial unlocking, go to the lock panel in the layers panel and click on one of the four lock options. You can then fully lock the layer or unlock the layer when you are finished editing.