Klaviyo automatically tracks profiles' first point of contact with your brand. This information is recorded in the How They Found You section on a profile page and can be used to build segments under the Initial Source property.
For example, you may be interested in knowing how a particular marketing initiative performed, so you can use segments to hone in on who visited your site and from which sources. If you use UTM parameters to track different marketing initiatives (like email campaigns), you can segment based on this information as well.
To identify these people, you can build a segment based on Properties about someone > Initial Source equals X.
If you add UTM tracking to your email campaigns or use UTM parameters to track your paid ads or other marketing efforts, you will see the names and IDs populate in the dropdown as well.