Adding a source to your notes is great for when you've collected a lot of information and you want to keep track of where it came from.
To add a source, select a text card and use the source icon in the toolbar.
Tip: you can use CMD-Enter to instantly create another text card under the one you're currently editing. Doing this will copy across the source of the card you're editing. This helps when you have a lot of data to input form the same source.