Audio in your project can be independent, linked or built-in. When an audio file first gets added to your project, it is always independent.
Independent audio - All new audio clips that are added to the timeline are independent. They are not linked to any video, and can be moved independently. Similarly, they will not be altered if you edit any other clips on the timeline. This makes these audio tracks perfectly suitable for background music.
Linked audio - If an audio clip is linked to a video, then they will stay in sync. If you move the video clip on the timeline, the audio track will be moved as well.
Now, the audio is linked with the above video clip. If you cut or delete the video clip, its linked audio clips (as well as titles and overlay clips) will also be cut or deleted.
Built-in audio - If a video clip has sound, both the video and audio will be added to the video track. The built-in audio is always together with its video, unless you decide to detach it.
To separate built-in audio from the video, simply drag the blue audio track from the video track below to the audio track.
To remove built-in audio from the video clip, simply select the built-in audio track and then click on the Trash icon.
Waveforms - The sound of each audio clip is represented by it's waveform. Those are visually represented on the audio track itself. Waveforms can be helpful if you are trying to find the quiet and loud parts of your video.