The Application Editor has a few tools to ease the building process, especially as apps can get complex when they have many pages, workflows, etc.
The Property Editor is the main panel that you use to design each page's different elements, and customize events and actions. It is a draggable popup that will show the different fields that can/should be filled for an element, an action or an event. It appears when you double-click on an element, or click once on an event or an action.
As soon as you have several pages and elements, you will be in a situation where you want to find all the elements that use the Current User's email, for instance, or all the ones that have a given style. The Search Palette provides a search tool within your application. Clicking on the magnifier icon in the top bar will reveal the palette. You can search for elements, actions, etc. Clicking on an entry will take you to the right page/tab in the Editor.
Since Bubble is very open-ended, you can (and probably will) make some mistakes as you design. A mistake can be a missing field or an issue with inconsistent type, such as using a text field type in a 'Charge a current user' action, when it should be a number for the price. The Issue Checker will check your app in real-time and list issues to fix. You will notice a red warning icon in the top bar as soon as you have an issue, and clicking on this warning icon will reveal the list. Again, here clicking on each entry will take you to the right place and highlight the corresponding issue.
It is important to keep the issue list as short as possible at all times (preferably to 0 items), as issues can make your app non-functional in run mode. You won't be able to deploy to the live, user-facing version of your app if you have issues (this is covered in Deploying an Application.
You can revert your app to any point in time, depending on what your plan allows. If something goes wrong, like if you delete a page that shouldn't be deleted and can't undo this action after a browser refresh, you can restore your app in the Version Control Panel. This will be covered in the relevant section of the documentation.