Contribution Source is automatically loaded with the contribution or event information for online payments. It's left blank for entries made by PowerBase users who are adding contributions "offline", which means it can be used to track specific groups of contributions. To do so, you would enter the same text in the Contribution Source for the group of contributions.
To find those contributions later, you can use the Contribution Details Report, or you can use the Contribution Search off of the Contributions Menu, or you can use Advanced Search off of the Search Menu.
When you search, you need to match exactly the source field unless you use the % percent sign as a wild card. For Example, to search for the Contribution Source of "Megan's House Party", you could enter %Megan% in the search field. If you have many different house party contributions and you want to find all the house party contributions, you could enter %House Party% instead.