So far I read the adobe help & premiere pro classroom in a book about targeting tracks with source track indicators & track headers. But I don't understand the difference between them, they sound like they do the same thing. Just looking for a simpler explanation, thanks.
They serve different but interrelated functions.
Track targeting affects which tracks in the sequence are 'used' for various functions, like Insert and Overlay, Copy/Paste, Next Edit shortcuts, etc.
Source routing has a more limited function in that it defines which tracks Insert and Overlay write to, but must be used in combination with track targeting.
For example to Insert video onto V2, you must both route the source to V2 as well as target V2. If you do only one, it doesn't work.
Things have changed a bit and simplified with the new version about to be released.