SUMMARY
Social Selling Index, or SSI for short, is a tool to determine your profile ranking on LinkedIn. The higher the score the better LinkedIn perceives you as a specialist user and the better reach you can expect.
You can check and track the changes of your SSI score in your dashboard in Prospect Labs:

IN DETAIL
SSI is LinkedIn's live and unique metric combined from 4 main pillars with a maximum score of 25 each. These pillars are:
👉 Establishing your professional brand:
Your position as a professional brand on LinkedIn. Start with your LinkedIn profile optimisation, ensure to have an active and well-presented company page and post & share relevant posts with your target audience in mind.
👉 Finding the right people:
Identifying the right target audiences using efficient search tools and practices. There are a number of different searches you can try to find the right leads, such as:
LinkedIn Events
LinkedIn Post Engagement
LinkedIn Groups
LinkedIn Search
Sales Navigator Search
LinkedIn Poll Scraping
👉 Engaging with insights:
Interacting with your audience on LinkedIn via polls, sharing chat-worthy insights, comment on other people posts that lead to conversations.
👉 Building relationships:
Connecting with the right people to grow your quality network.
To reach out to new leads you can run different campaigns in Prospect Labs:
Connector campaign
Group campaign
Open Inmail campaign
Import Connector campaign
Builder campaign
Remember that SSI is a live metric that changes constantly depending on your activity on LinkedIn, so you always have a chance to improve and grow your SSI score!