All Email Campaigns sent from Leadsquared would have an unsubscribe link at the bottom so that Leads who do not wish to get further emails from you could click on it and do so .
The re-subscribe button would let leads who has unsubscribed from your Email Campaign to start receiving your email campaign again. When a Lead clicks on unsubscribe they will see a page like below. They can click on re-subscribe if they have changed their mind.
Leads who have unsubscribed from your email campaign before and want to start getting emails again can click on Unsubscribe again at the bottom of the Email Campaign and this would take them to a page like below .They can then click on Re-subscribe to start receiving your campaigns.