Source field helps you to keep track of where the data was gathered from. For example, a source can be a cold call, Website, etc. You can also create filters using the source field to find accounts and contact records for a particular source. You can add a new source to the list as well.
Add a new source to the Source list:
1) On the home page click on the Settings icon
2) On the Settings page click on Sources under Customization.
3) Click on the ADD button at the up right-hand corner to add a new Source to the list.
4) Type in the new source name and click on Save
Now you can use the new source from the dropdown source list.