When you go to an active Campaign > Lead list you're given an option to filter your leads using a few Lead Status categories.
Here's what each of them means
Not Contacted Yet
This lead is yet to receive their first point of contact in a sequence.
In Progress
The lead has received at least one point of contact in a sequence. This could be the first email, or maybe a Social Platform profile visit.
In short, the atleast first step in the sequence has been completed for them
Completed
The lead has replied
The lead has completed all the steps in the sequence
Stopped
This means either of the below
Lead is blocked
Lead has unsubscribed
Lead got reply from the parent campaign
if the email sending fails we mark it as stopped (Even after retry)
Paused
You/Your team have paused the lead due to:
Contacting the lead from another avenue
Other reasons specific to your requirements
Blocked
The lead has unsubscribed
The lead has bounced
The lead has been added to the DNC/Global block list after being added to a campaign