Your Lead Sources refer to how your members heard about you.
When adding a new member, you can select how that member heard about you under the member profile (shown below).
This can be very useful when evaluating your various Marketing or advertising strategies to see which methods are most effective for your business.
The New Member Report in the Reports section will show you all new members within a given time frame along with their lead source. You can also filter the report by Lead Source to search for all members who came to you by one specific Lead Source.