Usually downline means those people that you get to join with your referral code, programs have different sized downlines, some give you a possibility to build a 5 tier downline (meaning if you get one person to join under you, he/she get one person to join under them etc...)
It also depends on programs how much you benefit from your downline, I'd say that most normal % that you can earn from your direct referrals is somewhere near 20%, so if your referral makes 1$, you get 20 cents from him/her.
You can start by getting a downline so that you start joining those programs that you arn't a member yet in the Downline builder so that you get 1 referral for yeach level sort of like for free

Then you can try to get more on your own either buying them, adverticing in other programs, getting friends and other people to join etc.
More info you can surely find in this folder!
Hope this helps you get started!
Pia