Objects may lie on top of other objects. When working with a single layer, you can change the front-to-back order of the objects in your drawing using the Bring to Front or Send To Back commands in the Home tab.
Any object that you insert into your visual can be moved along the Z-index on a single layer. The Z-index tells the app, whether something should be at the top or bottom of a stack within that layer.
Bring to Front moves any selected objects in front of the non-selected objects. Send to Back does the reverse. These commands have no noticeable effect unless objects overlap one another.
To bring an object to the front of the workspace, select the object then choose to Bring to Front.
Alternately, to send an object to the back, select the object and choose to Send to Back.
If you want more granular control over your objects, you may want to create multiple layers in your document. When you have multiple layers in a single document, you can move objects between layers and then change the order of layers as needed.