An Unsolicited Submission is a screenplay submission that is not coming from a professional source (an agent, lawyer, or manager). Many production companies and studios refuse to read unsolicited submissions, but they all keep a Submission Release Form on hand for when they get tempted by a new writer with a fantastic logline.
Keep in mind that many agencies and management companies also employ a “No Unsolicited Submissions” rule, so you need an agent to get an agent! The way around this is to network and meet repped writers so they will give you a referral to their agent or manager.
The common wisdom is that managers are more likely to read material from new writers than agents but it is my experience that they can be just as tough to get to. The first step is to win over their assistant and get them to accept your submission.