Sender Score is a number between 0 and 100 that identifies your sender reputation and shows you how mailbox providers view your IP address.
Mailbox providers take a lot of metrics into consideration to determine your sender reputation including spam complaints, mailing to unknown users, industry blacklists, and more.
Lower your Sender Score, mean you will have harder time for inboxing. Lower then <70 means you need a Big Repair, Greater >70 You are in line with the industry rules for email marketing, Above >=80 it means you are hitting the jackpot.