What does it mean when referrals are 'pending'

When people sign up for the Silver membership they are set to "Pending" and sent a welcome email. 

When they login to their membership for the first time it will ask them to confirm their membership level or give them an option to upgrade etc.At this point their membership is set to "Active"

If they never login to their membership to do this, then they will still show as 'pending'.