There are two ways to add your own notes. 

Add parts of the lecture texts to you own notes by highlighting the part you want and then clicking "Highlight". The text is now highlighted in yellow and added to your notes. If you wish to hide/show your notes, click Notes on the lower left part. 

You can also add your own notes on the video timeline. Click "Add note" on the lower left corner of the video and type in your note.
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